Vietnam doesn't have su-35s. It seems unlikely the most capable, fully operational, fighter in the Chinese arsenal will be relegated to OPFOR duties. I suspect Chinese su-35 will remain in front line service at least until a substantially larger number of J-20s have been in squadron service for long enough for the Chinese air force to have experimented with actual aircrews how best to leverage the unique capabilities of such an unusual ( compared to previous Chinese fighters) fighter.
The j-20's array of capabilities is so different from other jets, and each is likely such a national asset, that I suspect tje j-20 would not really be considered fully operational until the chinese has had chances to conduct many large scale exercises to explore how best to leverage its unique capabilities alone and in groups with more conventional assets, and develop tested doctrines on how to employ them.
Su-35, on the other hand, is just a point on a liner extension of capvilities and potentials the Chinese are already familiar with based on the large body of experience they've acquired from operating su-27 and variants for over 20 years. So su-35 can be quickly adopted into existing doctrines.
