It's a fine idea. Diminishing returns begin to appear at 40 for vitality, but sharp diminishing returns appear at 50, so you could go to 50 if you want to. Or not, if you feel you don't need the extra HP.
You stop getting stamina from Endurance at 40 so it's a good place to stop, although if you plan on wearing some of the heavier armor, you could really bump up that stat to the point of fast rolling. It could be quite the investment depending on what weapon and armor you're using. To put it into perspective, I needed 60 END to fast roll in the black knight set and with the black knight halberd. This was with both the Havel's ring and Ring of Favor and Protection. If you have yourself a good shield, you probably won't be needing to fast roll though.
Strength would depend on which weapon you're using. Some of the good strength weapons require high strength, like 50. But if you're not using those large weapons and are just in it for the scaling, 40 should be a fine place to stop.
