I always use a Versamark when using embossing powders as the vaseline content keeps it wetter longer thus holding onto the powders better. Plus as it's colourless, only the colour of the powder shows. Plus you then have the extra benefit of the 'watermark' effect if you leave it to dry. The other thing that's great about it is if you stamp onto vellum and you're not happy with the result, dry it off and stamp over it again until you're happy with the result rather than discarding it.
A bit long winded, but hope this helps.
Regards
Sandra