I can't remember if there is somewhere to buy vaccines that early if you go at first chance.

Yes, you can use it on enemies.... it is.... very helpful, but very cheap.

Nihp + 99 Chronus tears/Remedies = work my way through that dungeon at a very low level to get zodiac spear

Most boss/marks will only get slow, and maybe blind or something else from a remedy, but they do help.