Idealistically this would be true, but there is still the question to ponder... Are Sampires as you put it 20% better because they simply are, or because no one bothers to take the time to Min/Max another template towards competing with it?
The sad truth is, you can't balance UO, no player is locked into a rigid skill set structure. If you weaken one popular template then another simply rises up to take its place leading to more cries of "NERF NERF".
I remember years ago prior to Slayers and right after UO:R being introduced I ran a Stealth Mage. I could make my way through any dungeon, looting corpses of players and handle any monster that I came across. Magery, Med, Eval, Hiding, Stealth, Music and Peacemaking. I made more gold in a week selling silver weapons I looted off players and monsters, not to mention stuff on the ground in the back room of Brit Bank (I miss insecure trades), than I ever did with my Tamer or full blow Bard. They eventually put in changes that Nerfed that template to uselessness, I adapted. I swapped Music and Peace for other skills to round him out. Later on when Third Dawn hit, I found a new use for my Bard, I put mining on him. This allowed me to farm Ore Elementals that spawned at the time in Ilsh, and I made a fortune, was this over powered? I faced little risk, and made a killing. Changes were made to stop players from being able to do this, so I changed him up and re-worked his skills.
The thing is creative players will always min/max and swap skills around to yield the best results for both PvP and PvE. This happens in every game to an extent. Games where there is a set class people max out skill trees, or equipment, such as with SW:TOR. Games that are more flexible with character builds like UO they do equipment and skills. This is nothing new, and nothing that can be easily fixed, especially in the context of UO where any change to effect one template can negatively effect and unknown quantity of others.
The idea of changing something like VE to more of a leech model would definitely crimp a Sampires style, but what would it do to a Necro Mage? Are they OP as well? Should they take a hit for the sake of slapping a different template around with the Nerf Stick?