I would avoid SLU and Westlake, not possible to do anything quickly with a car at lunchtime.
I guess DT Bellevue might not be that bad if you can park at B Square - aren’t there usually some trucks on 106th & 6th? - , and I think some of the office buildings have trucks. My brother works at T Mobile just off I-90 and has mentioned food trucks being there.
Otherwise, in Seattle I’d suggest exploring the various Vietnamese delis in the ID or the food court at Uwajimaya (not sure if food court validates parking but the store does), Ezell’s on 23rd & Jefferson. Oh, and there is a food truck pod at Starbucks HQ in SoDo, that might work for you.
I can’t vouch for quality, but here are some schedules: