Acc. to the Lewis and Short dictionary:

vesper, ĕris and ĕri (in class. prose mostly acc. vesperum, and abl. vespere, or adverb. vesperi; the plur. not used), m. . . .

You can see examples of the use of both forms in the article linked to.

And, from the Oxford Latin Dictionary, just to be really nerdy about it:

Vesperi is an adverb, but in practise there is no difference in meaning. Both forms appear in Classical literature.