Happy to be corrected on this by someone who's actually looked into this properly....
You certainly could, under JAA rules, count Vig time towards maintaining a PPL if you had the TMG endorsement on an existing PPL. I'm not sure if you can count instructional hours towards PPL issue as most VGS instructors (probably) don't have TMG instructor ratings.
I haven't got experience of the NPPL and my student days were pre JAA so interested to hear if it's changed. Why don't you ask the guys at your local VGS or better still CGS as they will, I'm sure, have TMG examiners who should know chapter and verse on this one.
regards
BBK