If dirty tub can i get fungal vaginitis ?

Possibly: Fungi and bacteria can reside in dirt and dark moist places. If one were to take a bath in a tub with these growing in it there is a possibility that the open mucosa of the vagina could become infected. If you are having discharge, pain, discomfort, or any bad odors this might be the case. Best bet is to see your doctor who can determine what the infection is and how to treat it.

Not likely: Normal flora of micro-organisms in the vagina produces acid that controls other infections. Unless the tub is grossly dirty, that would not cause vaginitis in an otherwise healthy women. Incidentally douching is more likely to predispose you to vaginitis.