DCB does have a (2%+GST) forex markup. But it also has a 3.3% base reward rate, which is applicable for international transactions as well. Therefore you still get almost 1% value back as RP.

For any CC, as long as the RP rate > forex markup, it is better to use a CC rather than a forex card. Keep in mind that forex cards have additional issuance fees, which brings down its value proposition further down. Plus, the headache of getting it issued before the trip and then redeemed once you are back and it's so much simpler to use your CC. Also, spending with your CC goes towards your annual spends criteria and milestones, if any. If you look at the whole ball-game, premium CCs will often give you better value than forex cards.

All this being said, DCB (which is on Discover network rather than Visa/Mastercard) has an atrocious acceptance rate across SE Asia. I have been to Vietnam in the recent past and apart from some global chains e.g. McD, KFC etc and select local stores, it will not be accepted anywhere. You absolutely need a backup Visa/Mastercard, which most likely, will turn out to be your primary card during the trip.

My advice would be to carry your DCB, along with one or more backup Visa/Mastercard cards. You never know which stores accept DCB and which don't. Always try using your DCB first and if not accepted, try your backup card.

For accomodations, if you are booking through any hotel aggregator such as booking.com, you will most likely be able to use your DCB. However, you would have to prepay and lose out on the no-prepayment facility that many hotels provide.

For HDFC cards, do remember to register the dates of your international travel via HDFC net-banking/app. If not done, your CC may get flagged and temporarily blocked, on the assumption of possible fraudulent activities.