TRACON Controller reported their trainee vectored an aircraft that deviated from their course below the minimum vectoring altitude to avoid a conflict with another aircraft on approach.

Narrative

I was providing OJT on a sector; all control actions were taken by the CPC-IT. Aircraft Y was cleared for the Visual Approach and talking to ZZZ1 tower on the straight-in. Aircraft X was on the downwind for ZZZ also for the Visual Approach but not cleared yet. Aircraft X was issued traffic to follow on Aircraft Y. He reported it in sight. Aircraft X was issued traffic on an untracked VFR target which he also reported in sight. Aircraft X was instructed to maintain visual separation from the VFR traffic; follow Aircraft Y; and cleared for the Visual Approach RWY XX at ZZZ. A few moments later the CPC-IT noticed Aircraft X turning base INSIDE of Aircraft Y instead of following him. Although Aircraft Y was below the MVA the CPC-IT issued a diverging turn away and a climb. I concurred with the actions of the CPC-IT.I recommend in general issuing a base turn to a following Visual Approach aircraft to ensure they do not cut off other traffic. In this instance; however; and due to the uncertain nature of VFR targets I believe that using visual separation by the pilot was the best way to safely get around the traffic. As there are always numerous VFRs in the area I do not believe it would have been better to extend downwind because that could have led to more traffic problems.