According to the Magic World rules-as-written, "A vampire loses 2D6 Magic Points every night just to keep its undead body running." Later, when describing the vampire's bite, the rules state, "For every 2 points of CON drained, the vampire may regenerate 1 HP of damage." So, if I'm a vampire under these rules, my best strategy is to drain MP by touch from unaware or incapacitated victims; doing so, I'm unlikely to get damaged and therefore unlikely to drink blood at all.

Some vampire.

Looking at other rules derived from RuneQuest a/o BRP:
  • Basic Roleplaying states that blood drain converts STR or CON into MP. Vampires in the Gold Book regenerate 1d3 HP per round and lose only 1d6 MP when arising.
  • Vampires in Call of Cthulhu, 6th edition, have regular POW and presumably regenerate MP normally. Draining blood (STR) has no direct effect on the vampire.
  • Vampires in RuneQuest, 6th edition drain a Fatigue level per round, which progressively reduces all skill rolls until the victim passes out. Not consuming blood imposes a Fatigue level after each week. They're also "immune to most non-magical attacks, its flesh instantly re-knitting together or invulnerable unless specifically decapitated in a single blow". Fire, sunlight, and wooden stakes also work.

Of the various approaches, I'd suggest the Big Gold Book's; it's the most minimal change and forces vampires to actually drink blood. One could, however, change vampires around however one likes, even have different strains or species of vampires. I'd be tempted to write a book about it, if Steve Jackson Games and Eden Studios, among others, hadn't already done so.