Bingo.
Your not supposed to regenerate at all in sunlight. Its a bug that its letting you. Likely due to the regen effect getting placed incorrectly in the calculations for vampire charactes.
Uh....no, no it's not.
http://www.uesp.net/wiki/Skyrim:Vampirism
While outside during daylight, http://www.uesp.net/wiki/Skyrim:Health, http://www.uesp.net/wiki/Skyrim:Magicka, and http://www.uesp.net/wiki/Skyrim:Stamina regeneration is stunted by 100%. In other words, your health, magicka, and stamina will not regenerate. However, any effect that fortifies the regeneration of any of them will still work.
Regen and Recovery are set to 0%. Having bonuses on top of that will let you regen/recover again.