You can use magicka in vanilla as a vampire during the day with a slower regen off restoration etc, your saying ABSOLUTLY no regen what so ever from any source?
Sorry for caps on iPhone
As a vampire, or in vampire lord form you have a -100% magicka/health/stamina when in the sun.
The default regain rate is 100%, but if you have more regen (from robes for example) you will regain magicka in the sun. For example wearing robes that give you 50% magicka regen will result in 50% (150 - 100) when in the sun as a vampire.
Hope this helps.