How can I run 2" sink vent stack through top plate?
Depends what it's venting, which may be just the sink, or may be more than the sink, depending how things are presently arranged.
Per ICC, a kitchen sink (alone) is 2 DFUs, so if this is a dry vent serving ONLY the kitchen sink, if could be as small as 1-1/4&quot; (which is suitable for JUST 2 DFUs, no more) or it could be 1-1/2&quot; (suitable for up to 8 DFUs.)
However, your plan to expand to 2&quot; after making the corner (as drawn) fails 905.2:

Vent and branch pipes shall be so graded and connected as to drain back to the drainage pipe by gravity.

since the lower part of that upper 2&quot; pipe will not drain into the 1-1/2&quot; pipe because reducers are centered, not offset. At least all the reducers I've met are - could be someone makes them that way.