Re: worth it after sex??
If you have not been exposed to a certain strain that is in the vaccine, then you should have protection against that strain. You have to remember there are more than 20 different strains of HPV that can affect the genitals. The vaccine protects against 4 (2 of which cause 80% of genital warts and 2 which can lead to 70% of cervical cancers).
I'm not sure what you mean to "take effect." Once you are exposed to the virus you are "at risk of it taking effect."
Vaginal infections (yeast, discharges, etc) are not related with HPV.