When using Vray light material, how exactly does vray compute the sampling? Why doesn´t it have a sampling paremeter as the vray light?
Because it is essentialy a self illuminated material so you control the quality through the Irr map and noise threshold parameters..
Is that the same as store with irr map for vray lights?
Yep, pretty much, but the sub divs in the vraylight still have some affect on the quality.
the difference is:
vray light - is a light source which is understood by a renderer as point in space which emits direct rays and is not dependant on anything else other then itself.
vray light mat - is a shader, which depends on object to emit rays, and is dependant on the gi only paramiters. So it doesnt emit direct rays, it emits gi rays. As natty said, you can control the light mat through gi, and through qmc noise threshold.
I personally stay away form vraylightmtl whenever possible. You just have alot less room to maneuver settings wise.
indeed. i think vray light mat is there for those special cases, plus its a selfilluminated mat, so it can do some effects. Nothing more. i think whene vray light will support custom objects for shapes then vray light mat can be modified to be used as self iluminted mat only.
I use it quite a bit, instead of a max mat with self illum. But usually excluded from GI.
a nice usage of vray material by italian customer
take a look here there is someone who use vraylight mtl very good
http://www.treddi.com/forum/index.php?showtopic=15315
Wink
nice little article, but from what I could gather from the pics only, it kinda confirmed what I already knew: That in a comparison between GI lights and Vray lights, at an equal rendertime, usually the GI lights look alot worse and have really bad shadow definition.
I also do what rerender suggested alot: which is use it strictly for a self illum light only and exclude it from casting any GI.
excuse me percy but read better the comparision in rendertime between vraylights and vraylightmaterials to geometry and if u tweak a little with number of hsph and lower the interpolations in irrad map parameters especially in scene with high number of lights 
bye
your right. large numbers of vraylights really increase rendertimes where the GI lights comparitively dont. Which is best to use, is really dependant on your scene. Where I need better looking shadows, I just use vraylights, which is most of the time. We have a very pretty good renderfarm here, so longer rendertimes are not as big a deal as they once were.
hey so lucky u are 
but as always people haven’t renderfarm or dual core try to get the same quality as high parameters could give but in lower rendertimes as possible so those useful techniques born
Depends if you actually want any definition in your shadows though, all through that test I cant see any one where i’d pick the GIlights over the standard.
Shadow detail is one of the last things i’d try to cut down on when lowering the quality for a quick render.
Detail Enhancement fixes most if not all the detail issues with the light material precision (or QMC used as primary, of course).
It has some heavy speed price, but from a slow rendering to not being usable at all there’s a lot of gorund to cover (quick geometric lights sounds like an oxymoron to me…)
Lele