With most OTR microwaves, for the door to open, it takes more than the 30" of width. The hinge side of the door projects beyond the microwave side as it opens. So if the adjacent cabinet is too deep, the door won't open. Generally the door needs to be entirely in front of the adjacent door face. The vent and door are generally about the same depth, but the maximum cabinet depth requirement comes from the door swing.

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If you want a deeper cabinet, you'd have to block out behind the microwave to keep it forward. See these other discussions about it.

otr-microwaves-not-compatible-with-ikea-sektion-depth

Tight Microwave Install

For every inch your upper cabinets are deeper than 13", pull the microwave out 1". However, this creates a cavity behind the microwave which will need to be filled with a flame-resistant structure to which the microwave can be mounted. We use a simple plywood or OSB torsion box, screwed solidly to wall studs with the bottom edge protected with a sheet of galv. steel or aluminum. The OTR-MW then mounts to the box.

Check with your local code officials before you do this. They may want additional flame protection.

Is the vent on top of the microwave related to the microwave function, or is it part of the optional recirculation function?

I will most definitely keep the side cabinets the standard depth, but am wondering if the cabinet above it can be deeper, so that the cabinet face is flush (or close to flush) with the microwave. If this is a bad idea...would love to hear why specifically. :)