I've flown Vietnam Airlines (VN) many times and think they do a good job. So, given that it is your most prolific local carrier, I would stick with them and the Skyteam alliance. (If you were moving to SFO I suggest switching).
As to whether to book through VN or Delta, I would pick VN. Getting Delta miles is great if you fly Delta but because you fly VN I would keep the miles with them.
As to routing, there are only slight differences in the mileage. I suggest looking at the aircraft involved, the seats available and the timing. A middle seat on a 777 is worse than an aisle on a 330 and the few routing miles isn't worth it. Same for arriving at 6 am verses 6 pm.
One additional thought on routing - check KLM and AF. They may not be price competitive but every now and then they are and I expect that you would get additional miles going that direction. If it is substantially more expensive, but you really want the miles, look at buying two itineraries - SGN to BOM or DEL on VN and then BOM or DEL to PDX on KLM or AF. Just make sure to enter you Skyteam number when booking to get proper credit. Another option is to look at China Eastern via PVG which would fly you to JFK and then DL to PDX which would also add additional miles.
As far as hotels go, my advice would be to pick the chain that you frequent most - or that has locations in the cities you are most likely to frequent.
A final thought is to suggest that you check the mileage run discussion page. You may get some fine-tuning suggestions there as to maximizing your routing.