Can someone explain if there is a difference in meaning or usage between vicino and vicina. As far as I can tell, they seem to be interchangeable, yet I've seen both used.
Your confusion comes from the fact that vicino can be a noun, an adjective, or an adverb; vicina can only be a noun or an adjective, and it is the feminine version of that usage of vicino. In this sense, you also have the plural version.
Used as a noun, vicino means "neighbor" and vicina would be the feminine version of it:
Il mio vicino mi ha regalato una torta, la mia vicina invece è veramente antipatica;
used as an adjective, it means "close, neighboring":
La mia borsa è vicina alla tua, quei paesi vicini sono molto belli,
used as an adverb, vicino means "near":
La ragazza sta arrivando vicino al treno