CRJ-900 First Officer reported diverting to an alternate airport after experiencing high engine vibrations that led to inflight shutdown of the left engine.

Narrative

There was a maintenance delay due to the inbound flight reporting high N1 vibrations. The inbound flight reported N1 vibrations reached 1.9 VIB during cruise at FL340. However after descending it was brought back into limits. Maintenance in ZZZ1 conduced an engine run up at takeoff thrust and signed off the aircraft. The flight was uneventful except for a L FADEC FAULT 1 status message that came up twice. That was written up upon landing and MEL'ed. On the [next] flight; during climb; the left engine VIB entered the caution zone reaching 1.8 VIB. The Captain (PF (Pilot Flying)) elected to continue the climb with the left engine just outside of the climb detent and level off at an altitude below our filed altitude of FL370. During climb; ITT was slightly higher on the left engine than the right but well within limits. Oil pressure and temperature were both nominal. The Captain called for the QRH for N1 vibrations which we ran while climbing. The Captain briefed myself that during level off he would reduce thrust more than normal toward idle in hopes of bringing the N1 vibrations back into the normal range. Upon level off at FL330; the Captain reduced thrust as briefed; however the N1 vibrations increased and physical vibrations could be felt in the flight deck and through the flight controls; namely the rudders. Increasing thrust back to the normal cruise range had little effect on VIB although it did start increasing again. By then L ENG VIB was around 2.3. I snapped a picture on my phone of the engine instruments at this time. The decision was made to request a descent from Center in preparation for an intentional engine shutdown following the QRH which said that if vibrations were still occurring to proceed with the intentional engine shutdown QRH checklist. We requested FL250 and began a descending. We had agreed that ZZZ was our best option to divert and the Captain briefed the FAs (Flight Attendants) of the situation and ran through the [checklist] items. Following the Intentional Engine Shutdown checklist we coordinated with ARTCC for an uneventful single engine visual approach and landing. Upon clearing runway we had the fire team inspect the aircraft for any hazards prior to continuing to the gate and deboarding passengers.When we accepted the aircraft from Maintenance we discussed that there was a likelihood of the incident happening again. Maintenance advised us that the engine 'probably needed a fan adjustment soon' but they signed it off so we accepted the aircraft. Maintenance procedures may be inadequate to troubleshoot and sign off an aircraft that has had N1 Vibrations in flight. Especially in this cause where the initial vibrations on the inbound flight happened at FL340.