Between 1450 and 1580, Portuguese trade was dominated by the Low Countries, particularly trade with Antwerp. There the Portuguese marketed their spices, and borrowed funds from local financing syndicates. The borrowing was so heavy that the Portuguese Crown had to renegotiate its debt in 1554. Also, the trade tended to benefit the Low Countries as the Portuguese purchased manufactured goods, luxury items, grain and naval stores from Northern Europe. With the rebellion in the Low Countries against Spanish rule, followed by a suppression, the Portuguese trade had begun to disperse to Holland and Zeeland by 1580.
Until around 1550 the Portuguese brought spices to the Low Countries to market them, establishing a trading house at Antwerp. Afterwards it was mostly shipping from the Low Countries coming to Lisbon to purchase them. The wars between the Habsburgs and Valois between 1551-1559 made it so that French corsairs preyed upon neutral Portuguese shipping to the Low Countries. The Portuguese protested to the King of France as grain from Danzig brought by Flemish ships was vital to Portugal. After 1550, Portuguese ships rarely ventured North of the Bay of Biscay. The Baltic Trade was exclusively in the hands of the Flemish and later Dutch merchants, bringing naval stores, timber, copper, lace, and manufactured goods to Portugal. The terms of trade largely benefited the merchants of the Low Countries, as Portugal bore the costs of maintaining 15,000 men under arms and a far flung string of fortresses and trading posts, stretching from Lisbon to Japan.
The Flemish and Dutch merchants also needed salt to preserve herring. The salt from Portugal was considered the best for this task, and it was a major commodity in trade. In 1598, an embargo was imposed on Portuguese-Dutch trade, and it was the result of this embargo that the Dutch went looking for salt. They headed to the salt pans of Punta de Araya in present-day Venezuela, establishing their first trading links with the New World. Between 1598-1605 some 800 Dutch ships visited the area, bringing back tobacco and other goods as well. Another important result of the embargo was the issuance of "letters of marque" by the stadtholders of the various provinces against Portuguese and Spanish shipping, making them targets. Additionally, the Spanish Crown had disallowed its realms to trade with the rebel provinces, making the Portuguese merchants have to trade Hamburg instead, and depriving the United Provinces of spices and sugar. The result of this was that the first Dutch expeditions to Asia around 1595, culminating in the formation of the VOC in 1602. Its initial actions were largely to attack Portuguese ships, and the capture of Portuguese ships in the East proved to offer a big return on investments.
Eventually the first trading posts were formed, including Batavia on the island of Java in 1611. The signing of a Truce in 1609, led to a period of peace for the Portuguese shipping and allowed the 1610s to become the peak of Portuguese-Asian trade. One of the terms of the Truce, was that the Dutch also guaranteed that they would not establish colonies in the New World, and they did honour this stipulation. However, in 1621 when the Spanish refused to renegotiate the truce, Portuguese shipping was attacked with much more ferocity than previously. Among the first actions was the founding of the WIC, or West India Company to attack Portuguese and Spanish shipping and possessions in the New World. This culminated on attacks on Brazil in 1624, including the capture of the capital, Salvador for a year. This was significant, as Brazil was the world's largest supplier of sugar, and the Low Countries were a major sugar-refining region with Amsterdam's number of refineries increasing from 3 in 1600 to 25 by 1622. By 1630, the Dutch were able to capture Recife, a major sugar-producing region of Brazil.
Additionally, after the renewal of hostilities the Dutch once again needed a good supply of salt, and St Maarten was settled in 1631. The need for salt was also the reason for the colonisation of the ABC Islands, but these soon proved to be more lucrative as places from which to smuggle goods to the Spanish Main. Importantly, the capture of a Spanish silver fleet at Matanzas near Cuba in 1628 gave the WIC the revenue with which to attack Spanish and Portuguese shipping.
With the overthrow of the Habsburg Rule in Portugal in 1640, the Portuguese were eager to sign a truce with the United Provinces, and this was signed in July 1641 and ratified in February 1642. The 10 year truce, was signed, but its news would only reach the colonies months later. In the interim, Galle, Malacca, Luanda, São Tome and El Mina had been captured by the VOC and WIC. The Portuguese were still fighting with the Spanish, and therefore did little to protest. When the truce expired in 1652, the Dutch were busy at war with England and the renewal of the truce negotiations dragged on. As a result, in 1653 a Portuguese fleet was sent to Brazil to recapture Recife, with the last Dutch surrendering in 1654. The negotiations now centered around compensation, as the WIC wanted monetary compensation for the loss of Brazil and Angola. The Portuguese were unwilling and unable to compensate the Dutch, leading to a renewal of Dutch privateers against Portuguese shipping. A treaty was concluded in August 1661, but during the interim, the Dutch had captured the Ceylon, holdings in India and in Makassar.
Prior to 1580, the primary interlopers in Portuguese trade had been the French who had attempted to establish colonies in Brazil. The difference was that as Portugal and France were not at war, these were done in areas not under Portuguese control. Also, French privateering had been the major concern, and the letters between the Portuguese and French diplomats show that the French Crown and its authorities often feigned ignorance and promised to punish the privateers and compensate the Portuguese. As Portugal was at peace with France, this policy seemed to be effective. However, by the 1550s the Portuguese complained of French traders in Guinea, though these were in areas often ignored by the Portuguese, such the Pepper Coast.
During the reign of Elizabeth I, up until 1580 the relations with Portugal and England had also been cordial, and the Portuguese complained about English ships trading in Guinea, and the Barbary Coast. The result was that in 1576 a treaty was signed where the English agreed not to trade with areas under Portuguese influence. The motivation seems to have been that the Bristol merchants felt that buying the goods directly from the Portuguese, at least in the interim was more profitable, particularly as prices of spices had become lower as a result of a renewal of the Levant Trade via Venice. Significantly, the treaty of 1576 was the first of many between the two countries where the English merchants were able to export higher value manufactured goods to Portugal, much to their benefit.
If there is no Union of Crowns, I imagine that the Portuguese possessions will not directly be attacked, but probably be bypassed as the French and English establish their own. Whether or not the Dutch feel the need to do so is up to anyone's guess. The initial capital for the VOC came from privateering issued against Portuguese shipping, something which had not occurred prior to 1580. Seeing as the VOC was at war with Spain, it would probably not want to attack a neutral country from which it enjoyed a beneficial trading and financial relationship. For instance during the 1609-1621 truce, Dutch trade with Portugal flourished, and Amsterdam would remain the leading port of trade with Lisbon until the end of the 17th century. Though the VOC was profitable, the WIC went bankrupt in 1636, and had to be recapitalised several more times. The need for venturing to South America is also butterflied away as the Portuguese were only too happy to sell both salt and sugar to Holland and Zeeland.