Is it safe to swallow semen after vasectomy?

No, unless you are both tested and clear from the possibilities of having STDs-

otherwise certain STDs such as herpes can be transmitted.

Females do not create semen. However, they do create vaginal discharge, which can spread diseases as the above contributor said. Dental dams (for oral contact) and condoms (for penile contact) can be used to prevent disease, but be careful.