There are brown examples of NERD/VPIs, and very clean, silver examples of both TSK and Joliff. (LadyOhh has blogged about this with side-by-side pics.) Really, the only way to be certain, if you can't trace the lineage, is to get a VPI het and a TSK het and find out which breeding produces axanthics.

Joliffs do seem to be much less common? And most people tell you right up front if it's a VPI line, because it's considered more desirable. So my first guess would be TSK. But it's not certain until compatibility is confirmed.