Why didn't Accelerator get sunburned after losing his vector shield? Albinos are very vulnerable to ultraviolet, so he must have been red all the time.
It wasn't explicitly stated, but the collar has three states: Active, Passive and Dead Battery.
In Active mode, Accelerator can use his vector transformation powers. In Dead Battery mode, Accelerator can't walk or talk, he can barely think logically, until the battery is recharged or the source of electromagnetic disruption is removed.
In Passive mode, Accelerator can think, talk and walk (although with a cane), so it isn't a stretch to think that he also has a weak enough reflection shield to keep protecting him from UV light.
Also, another possibility is that the author never thought about it.
From what I understood it's not that he can't use his power. It's just that the amount he can use is limited because his calculations ability is decreased. So he should still have enough to do some stuff. But if he wants to fight with it he needs the collar to help him calculate.