welp, there's pretty much no way necrozma isn't winning this, but I'll try my best.
I don't know enough about necrozma's absorption, can they just absorb any energy or does it specifically have to be light? Also does absorbing stat amp or heal necrozma?
VTSG's first most is to immediately summon 4 planes which are going to be constantly fire homing darts and bombs directly at wherever necrozma is at any given time. VTSG will also be summoning dark avatars that get passively amped by being in VTSG's presence, however if Necrozma can absorb all their attacks regardless of AP or intensity then that may end up backfiring. (the Avatar's attacks should be at least as hot as the core of the sun, likely more)
heat manipulation. Super Monkey gets turned to vapor.
speaking of this, how hot is Necrozma's heat manipulation? Also how much heat manipulation can Necrozma resist? Because most btd characters are shown to casually exist in plenty of extreme conditions without any sort of protection and be just fine, including fighting bloons while suspended over an active volcano. On the flip side, as I mentioned the dark avatar's plasma attacks should be at least as hot as the core of the sun on their own, plus they're getting constantly stat amped. The Vengeful True Sun God's main plasma attack should be even at least as intense as the dark avatars, likely far higher.
In addition to that, if Necrozma can't be harmed by all that stuff, there's still the bombs from the plandes as well as VTSG's own range of attacks to add to the danmaku including a ring of blades, glaves that as soon as they hit a target immediately ricochet directly to another target and repeat this until it hits the pierce cap, glue attacks which will debuff Necrozma's speed, ice manipulation which can freeze bloons that don't resist ice manipulation fully in place leaving them as free unmoving targets, missiles which actively do more damage against bigger enemies (i.e. moabs in btd6), pure arcane magic attacks which also have homing on them, and a magic field of knockback that for some reason is listed on the character page as a forcefield which instantly knocks everything inside it away. Speaking of knockback, even if Necrozma absorbs VTSG's plasma attacks, they should still at least be influenced by VTSG's distraction ability which uses some kind of magic mind manipulation to make necrozma run directly away from the battle for a short period of time. In that time, they would be an open target.
That's everything from off the top of my head, there's probably something I missed
