We travelled to Vietnam when DS was 15 months old.....

We got an overnight flight to Singapore where he slept most of the way. He was ok in the bassinette - not too big for it. then a couple of hours at Singapore airport - we let him run around and burn off energy - then he napped on the next flight.

I'd still request the bulkhead even if you think he'l be too big for the bassinette. It'll be good to have a little xtra room to get in and out when he wants to go for a wander.

Can I just add that one of my fondest memories of the trip to Vietnam was when DS spent agood part of a two hour flight wandering up and down the aisle (DH following) giving everyone high 5's and then giving everyone high 5's as we got off the plane

Oh, and be prepared to be totally wiped out when the journey finishes. If you can get someone to be able to babysit or play with your child when the journey finishes. You'll need the rest