I do know that if you hold an SLMG rating and have 75hours total SEP which includes 25hrs SLMG you can apply for a TMG rating on either the old CAA or JAR licence. If you do not have the 75hours SEP/SLMG then , as you surmise, 75 hrs on SLMG counts instead. You do not say if you are flying the Vigilant on an SLMG rating or on a service equivalent - if the latter then I would check with the CAA. The TMG rating was, for a short period, added free of charge but I believe that you now have to pay.
Yes - the hours for "revalidation by experience" of your SEP can be done entirely on TMG but remember the requirement to have done a 1 hour training flight with an Instructor and have this signed off in your log book prior to having an examiner revalidate the ratings.
Keep in mind that, as far as I am aware, only hours flown after gaining the TMG rating will count for revalidation as those flown previously would be under your present rating and would not be acceptable for your SEP revalidation.
PS These requirements are likely to change when EASA licensing comes in and you may have to do at least one flight each in an aircraft matching each of the ratings within your licence but this is still in the melting pot and several years away.
I hold both SLMG and TMG ratings on my CAA PPL having obtained the latter free several years ago