Porgyon 2 as a prestiger

In theory it should be good against Vaporeons and other water types, possibly even against Dragonites.

Porygon2 is not bad. But there are better options.

Jolteons are easy to get an will get the job done.

Vaporeon and Omastar

Mine has Grass Hidden Power + Solar Beam and I use him against Vaporeon sometimes, he's pretty good. The same goes for yours, since Charge Beam is also SE against Water and it charges the deadly SB super quickly.

What imo makes a good prestiger is a good rate of "damage done until it faints" and CP.

As defense and stamina are square rooted in CP-calculation and go directly into damage until it faints, good defense and good stamina are the first point to check. Porigon 2 is good here, but not overwhelming.

Second aspect of attacking power that does not affect cp is moveset. Three points here:
(1) STAB
(2) Super Effective
(3) High damage output.

Your Porygon lacks stab, and this missing 25% makes its prestiging performance underwhelming in comparison to real grass prestigers like venussaur or tangela or even SoB-parasect.