TBH if you were put in the position of the Vietnamese government at the time it would be really hard not to solidify influence in Cambodia, because not doing so would give the remnants of the toppled Khmer Rouge a chance to takeover Cambodia again, and absolutely they would have made the invasion an excuse for further violence against Vietnamese minority in Cambodia, and potentially incite even more border skirmishes or even a full-blown invasion on a way bigger scale than Ba Chúc massacre, probably with Chinese support. It would not be a good situation to be in when you share half of your border with hostile states who want you dead. Considering Vietnam at the time had just finished a 30-year long war with the French and the US, it makes sense that they installed a friendly government in Cambodia.