Center Controller reported vectoring a departing aircraft below the Minimum IFR Altitude to avoid a VFR aircraft not in communication with ATC.

Narrative

Aircraft X requested IFR off AST back to ZZZ climbing to 050. On departure there was numerous targets we weren't talking to that affected their climb. They appeared to be flying the ODP (Obstacle Departure Procedure); which was not assigned (but obviously their prerogative). I gave two separate traffic calls starting around 010. The second traffic call was for an aircraft with a specific tag about 7 NW of the field out of 024 descending. As Aircraft X continued to climb; on the second time I issued traffic I deemed it necessary to give a traffic alert. they were out of 019. I said 'traffic alert; Aircraft X; traffic 1 o clock; 3 miles; two thousand one hundred descending; advise you turn to the north; maintain vfr'. They did in fact turn to the north. Obviously I shouldn't have said 'maintain vfr' because they were IFR. It was just reactionary because they were below my 030 MIA (Minimum IFR Altitude). Once they were clear of traffic; I said 'traffic no factor; resume on navigation; climb and maintain 050. I am familiar with the area's terrain; and also knew that the plane to the NW of the field was over a very lowland marshy area and ultimately turned over the Columbia river mouth that is barely 50 feet above sea level. traffic alert and advising a turn below MIA seemed safer than a near collision with an aircraft we weren't talking to. It would be nice to have some more micro MIA's for areas near airports to be able to assist aircraft better and more legally on theses departures. With ADSB now; we see so much more traffic that we aren't talking to; and it's our responsibility to maintain safety; which is difficult to do; when the MIA is so high for what we can know is such a low area. I understand that could cause more scope clutter and has drawbacks; and also know that to the east of that airport terrain does get high quick;. There are many situations where we have few tools to make the safest decision in near airport phases of flight.