So, We were recently fighting a "monster" wich was using a playable race (no LA) and had levels in druid. How does one calculate exp for this? I was also fighting a vampire, at what stage do you use CR increase for vampire template and when do you use LA? Of course both on Playable races/classes that we as a party are fighting
An NPC never cares about LA. Only CR determines experience rewards. Consult this question for how to figure out an NPC's CR.
As per Improving Monsters:
Most templates increase the creature’s Challenge Rating. A template might provide a modifier to be added to the base creature’s CR, or it might specify a range of modifiers depending on the base creature’s original Hit Dice or CR.
While an NPC technically does have an ECL (which is what LA is factored into), the NPC's ECL never matters. From reading the monster entries:
A character’s ECL affects the experience the character earns, the amount of experience the character must have before gaining a new level, and the character’s starting equipment.
An enemy NPC doesn't care about any of these things, since he doesn't earn XP nor level up.
\$\begingroup\$ Oddly, the DM's supposed to use a creature's ECL (not CR nor actual level) to determine the gear for a monster with class levels (MM 291). That's madness, of course, contradicting the DMG, but a killer DM can make a thing of it. \$\endgroup\$ Mar 7, 2016 at 17:40
\$\begingroup\$ That doesn't seem all that useful to a killer GM, since monsters with LA -- have an ECL of --, which bestows no gold whatsoever upon the creature. \$\endgroup\$– SPavelMar 7, 2016 at 19:06
\$\begingroup\$ But for those monsters that do have a listed LA, it's an impressive windfall. For example, a frost giant is normally CR 9 and has treasure appropriate for that, yet a frost giant War1 (also CR 9 because warrior levels are nonassociated) has that appropriate treasure plus gear for a level 19 NPC. \$\endgroup\$ Mar 7, 2016 at 19:16
\$\begingroup\$ A GM that is killer enough to do this, but not killer enough to abuse monsters who don't have LAs but do have nonsense super-powers (like Chronotyryns) is a very rare beast indeed, I think. Why deprive your Chronotyryns of any wealth at all just to give some lousy giants loot that the PCs will find some way to steal anyhow? \$\endgroup\$– SPavelMar 7, 2016 at 19:58