Can I mix liquid fabric softener with vinegar if yes what's the ratio?

You can--but why do you want to.

You can use liquid softener and vinegar, but I would not mix them. If you are putting them in the washer I would put the softener in the compartment designated and the vinegar either in the soap dispenser or in the bleach dispenser. I am assuming you are using the vinegar to take out some of those musty smells.

Curious as well what the use is?

I would just use vinegar and water to reduce odors in your home. If you want to tuck some fabric softener sheets here and there--that also helps with odor. I don't know that liquid fabric softener would do much for you.

I use just vinegar. I stopped using fabric softener a yr ago. My towels were not smelling as fresh and they were not as absorbent as the once were. Apparently they get a build up from fabric softener - especially from dryer sheets. Vinegar really cleans them. They smell fresh and are softer and more absorbent than before.