Would there be any advantage in plugging in a mic into a Focusrite or similar interface before the Zoom r24. Would it improve the vocal quality.
Or any best set up tips on recording vocals.
There shouldn't be a difference since going into the Zoom will use the mic preamps anyway. The only variable between the line and mic signals is the gain pot. Unless the Focusrite is coloring the sound (doubtful), the only other change should be the amount of available gain.
I've done quite a few vocals straight into the R24. The only time I've used something prior to the mic input was a Yamaha mixing board feed from a PA. What particular problem are you having with recording vocals? If its insufficient volume, you might give us more information regarding the equipment you are using (specifically the mic and which Focusrite unit you have).