We are thinking of possibly spending about 3 days on Koh Mac Island sometime in our 2 months Thailand/Vietnam trip. Any comments or advice would be appreciated. We would be looking for inexpensive lodging. Clean and safe otherwise just some quiet beach time. Is Koh Mac a good place for that?
Warrsher

I think you mean Ko Mak. (Haven't been there.) Google search for Ko Mak will give you some pointers and posts for hotel advice. Looks like November to March is best time to visit.
Koh Mak: Thailand's Best Kept Secret | Good Time Resort Koh Mak

Sounds like a nice peaceful place to visit. (I am not a beach person and prefer something more lively.)

Have a nice time on your trip.

Ko Mak is a community based agricultural landscape island which is different from main stream Tourism.

a Clean Green Environment..... backing long beaches and several satellite isles for day trips.

ENJOY BUDDIES ...

I am pretty sure we visited Koh Mak from Koh Chang when doing some diving in the region but it was many years ago. Several nice unspoilt islands in the area and I am sure KM is one of them, but Koh Chang has long been over developed.

My only concern would be the time it takes to get there. You will likely need a bus, minibus, ferry combo from Bangkok but it could work well if you plan on entering Vietnam via Cambodia ( which I would recommend if planning time in the Mekong Delta