Agree with delaying evolution for Razor Leaf, as it's streets ahead of Vine Whip.
Giga Drain only has 60 base power and 5 PP in Gen 2, to Razor Leaf's 55 base power & 25 PP. Basically the same damage, but 5x the PP for Razor leaf.
Personally, I'd keep Razor Leaf over Giga Drain if you had the choice, unless you're confident that Victreebel's other moves will make up for the low PP on Giga Drain.