If you dive no-stop, you most probably won't need a heated vest. If you can avoid clutter and tech which can fail, avoid clutter and tech which can fail.
Me, I have two undersuits. My summer undersuit is pretty light. Works in 10C water or warmer. My winter undersuit is heavy and works at least down to 3C water. Both are fleece-based. I used to have a padded undersuit, but IME the fleece undersuits are more elastic and more comfortable to wear. No matter what, wear wool as your wicking layer (have I mentioned that before? Yes, several times)
When I'm in my winter undersuit, I carry some 4 kg more weight. Because buoyancy. I dive no-stop and have never missed a heated vest (nor a pee valve, but that's an entirely different discussion and not relevant here). But then I'm not tuning my weighting as low as possible. Weight = warmth.
And a good hood makes wonders. Your brain is your most valuable organ, so your body will protect it at all costs. Even if that means sacrificing everything else. Keep your neck and head warm, and you'll have a lot less issues staying warm in general.