/g_vampire <number.dec> - If you set this value above 0, you will gain health when inflicting damage on an opponent. For example, if you set it to 0.3, you will get health for about 1/3 of the damage to your opponent; if you set it to 1.7, you will get more health than the damage to your opponent, and with 1.0 you get the same amount of health as the damage caused. Be careful, since if you hit a corpse (the dead body of someone already killed, but not yet disappeared), you will receieve damage (you will lose health instead of gaining it): randomly spamming around with explosive ammo may be dangerous (and in particular, may be a major problem with proximity mines). Default value is "0.0" (disabled), and usually (if you want to use vampire mode) it is advisable to use values lower than 1.0, e.g. 0.3.
/g_vampire_max_health <number> - When /g_vampire is used, maximum health value is customizable using this CVAR. Default value is 500. Note that this should not be set higher than 998, because 999 and over counts as infinity and the player will become impossible to kill. Another "risk" related to this cvar is to fall in a deadly pit that gives 999 damage and survive because you had more health, remaining trapped there until you suicide (/kill): to be absolutely sure you cannot survive to a fall that should kill you, this cvar should not be set over 798, because 799 (max vampire health) + 200 (max armor) = 999. Anyway, this risk exists only in few maps: in a correctly programmed map, being at the bottom of the deadly pit should continue to damage you, killing you in a short time anyway.
This is currently already implemented in OpenArena gamecode
From OA Wiki: