As we know US left a lot of hardware with south Vietnamese forces that were captured by communits
In otl most of this became inoperable quickly due to lack of spares
How can we have them equip several divisions and squadrons of the Vietnamese armed forces ( ensuring a decent supply of spares etc) and use them perhaps to completely rout the Chinese in the sino Vietnamese war
Logistical support from other nations in terms of spare parts and other technical support, including refurbishment and maintenance. I actually am a native of the region and know something about the history.
But very unlikely though, in my opinion. At that time, the other countries of Indo-China, all the way down the malaysian peninsula, saw the Sino-Vietnam war as sort of a mixed blessing, because it took valuable Vietnamese resources away from the occupation of Cambodia; back then, there were reliable indications that the Vietnamese military were preparing to cross into Thailand (although everytime I state this, i am lambasted with howls of derision ).
In an ATL, one alternative way would be for the both circumstances to happen:
1) for the other nations, particularly Thailand, Malaysia and Singapore, to have much better relations with Vietnam's communist leadership. In OTL, all 3 nations were treated with great hostility by the newly-reunified Vietnam till the early 1990s
2) the invasion by China was seen as a prelude to a massive land invasion of all of Indochina by China, although this may require a much bigger butterfly effect. In OTL, China's leader was Deng Xiaoping, whom reached out to the other nations of Southeast Asia to build better ties. Maybe a different leader, with more hostility towards these nations? If so, then China would be seen as the greater threat.
Even then, the role of the US is critical. In OTL, during the early 1990s, Singapore's state-owned defence contractor actually got a contract to refurbishment and upgrade Vietnam's M113 APCs, which were from ARVN. Singapore had expertise as it also operated US-made M113. It was a low profile deal, but the US got wind of it and applied pressure to Singapore to cancel the contract, perhaps because the technology used for the refurbishment and upgrading was of US origin.
So in this ATL, even if the other nations were willing to provide maintenance support, they need to factor the US government in as well.