Is it ok to start a vent on a 45 from a wye like on this drawing?
I need to clear the edge of the sink (actually a giant double compartment sink) before I can go up and start horizontal once I clear the flood rim + 6".
Absolutely! Most any fitting is perfectly acceptable and legal, just don't use any reducers, caps or traps in the venting.
Though, check with your local Codes, Building, Zoning, Permit or Inspection Department (whatever they're called around you). That 6-inches above the Flood Rim may need to be 36-inches and if there's a toilet on the line a 2" vent pipe may be required. Same goes for bird screening or goose-necking of the exterior pipe termination.