[Varyon]

Blood is roughly 1 kcal per mL, and GURPS characters eat 3000-3600 kcal/day (of those rations that can be calculated, only the pemmican and jerky in LT fail to meet this level, but jerky rations in all other supplements do). Thus, an SM+0 creature with Restricted Diet (Blood) and no Reduced Consumption or similar would need to consume 3 liters of blood every day. That's more than half the blood in a typical person's body, and above 40% blood loss is a Class IV hemorrhage, which is basically guaranteed death without immediate medical assistance. Based on some old work I did for bleeding in GURPS, every liter is worth somewhere around 12 HP in injury. On the bright side, with blood being mostly water, 3 liters of it should cover your daily water needs.

Unless you want your vampires to basically need to kill someone every day (assuming they need human blood, as otherwise they can use cow blood or similar), you'll either want to go ahead and give them Doesn't Eat or Drink and some relevant Dependency or similar, or you'll at least want to give them several levels of Reduced Consumption. In order, from Reduced Consumption 0 to Reduced Consumption 4, this is 3 liters (~36 HP), 2 liters (~24 HP), or 1 liter (~12 HP) per day, 1 liter per week (~1.7 HP per day), and 1 liter per month (~0.4 HP per day).