The violin is like the trombone,in that, to play in tune you have to listen to the sound you make and correct it very quickly if you are out of tune,you have to be able to hear whether you are flat or sharp.
It requires completely differnt learning skills to a fretted or fixed note instrument.
Fiddlers can also play quarter notes[notes between say g#and A]if you put frets on a fiddle ,this will be lost,fiddle players can also play in mean as well as equal temperament, [important for early music]this too will be lost if you fret your fiddle,fretted instruments are designed with equal temperament in mind[that is how I understand it].
in my opinion if you cant play in tune on a fiddle,you need to get a fiddle teacher to spend hours developing your ear,or play a lot with a fixed note instrument.
Vibrato is debatable,it is a matter of taste in traditional music,personally I like it if it is not overused.but to have an istrument that limits you from using it,would not suit me at all.