What about a POD where Portugal builds a fort and trading post at the Cape in the late 1500/early 1600s and the Dutch capture it and the VOC/VOC analogue decide to settle more people there to prevent a Portuguese recapture during their wars with Portugual in the first half of the 1600s?
If history is any guide, the Portuguese would send far more settlers to colonise the land as they did when the founded Luanda. The most opportune time for the Portuguese to build a settlement would have been 1500-1530 when they were flushed with cash. With a better climate they would probably multiply, and it is not implausible that 100,000 Portuguese could live there within a century. One of the major contrasts between the Portuguese and Dutch colonisation is that the former for the most part resembled a quasi-feudalistic rather than capitalist mindset. One of the contemporary criticisms of Portugal from this period is that the ruling classes reinvested most of their profits from trade in Asia in land. This concept of acquiring large tracts of land extended to Brazil. Hence the Portuguese crown dumping peasants all over Brazil and all over the coast of Africa throughout the 16th to 18th centuries, and while many were degredados, large numbers came from poor areas with little other opportunities. The numbers arriving were also much larger, and though in tropical areas the numbers of Portuguese deaths often exceeded births, the result was often a mixed-race community that was often Catholic and spoke Portuguese or a Portuguese dialect, and in some regions such as in Africa or Asia, allowed Portuguese sovereignty to endure.
The VOC could take a coastal fort or settlement like they did with Brazil, but again in Dutch Brazil their population. The Portuguese would probably settle there under Dutch protection, so that 1,450 could be present by 1645 and fewer than 600 by 1654. There were some 2,000 Dutch/Germans/Scots in Dutch Brazil at its peak, but this was minuscule compared with 25,000 Portuguese under their rule. The planters borrowed heavily from the Dutch and Jews and decided to overthrow them rather than pay their debts. Even Brazil, which was so attractive to the Portuguese as a place to settle, seemed to attract few Dutch. The VOC is richer than the WIC, so it might be able to hold a Portuguese populated Cape. The question if they might think it is not worth it. The colonies they did take from the Portuguese were always thought of in economic terms. This was in contrast to many of Portugal's colonial ventures during the period, which were often unprofitable, at least for many years. The Dutch Ambassador in 1677 wrote disparagingly of the Portuguese recruiting settlers from the poor and shipping them off to an unknown fate "in chains".
Northeast Brazil = Sugar
Angola = slaves for Brazil
El Mina = slaves and gold, dyewoods
Ceylon = nutmeg
Malaca = tin
East Indies = spices (cloves, etc)
Malabar India = pepper
Unless gold were discovered before 1660, the colony would probably only produce hides and skins, grains, perhaps some wine. The VOC did feel the need for a way station, but as the Cape was unsettled, it was a good choice. A cost benefit analysis by shareholders would probably lead to the conclusion that Madagascar for instance might be a good choice as it could also grow sugar. Any invasion of a Portuguese Cape would require a costly expedition force of ships and men, with little in return. This was in contrast to Brazil, which at the time of the Dutch invasion produced 90% of the sugar consumed in Western Europe.
Additionally, the Netherlands in terms of attractiveness was pretty high for Europe for places to live in Europe during the 15th-18th centuries. The wages there even for agricultural workers were much higher than those found in other parts of Europe. The government was fairly tolerant of religion, and even prior to the Dutch Revolt the area was among the most literate and industrious in Europe. A VOC conquest to out-populate the Portuguese population would require a large amount of capital, with shareholders unwilling to put up such capital. Portugal by contrast was a largely unproductive country unable to feed itself with little industry and being far away from wealthiest parts of Europe, limiting migration there.