Later versions of CoreTemp work fine on Vista x64. I remember having problems installing an earlier version of CoreTemp, but am currently using 0.97.1 without problems on Vista x64.

As to the max 'safe' NB voltage on stock cooling, it all depends on how effective the cooling solution is. I personally wouldn't be worried about using 1.5v on a P35 chipset, although I would monitor the temps. If the temps do get a bit high, you could always either add a fan to the NB area or consider after-market cooling solutions.