Yes, that does make quite a difference!!! I think that amount of cash would be ok off-season in Vietnam, but it would be quite a stretch in high-season Dec-Feb. In either case, you'd be staying in the cheaper hostels. Look at the listings at hostelworld.com for prices.
Those who have been to this two country, is SGD1000 enough for about 1 month trip? Excluding airfare.
Your first post states 25 days in either VN or Myanmar with a budget of $740USD. You will be able to find a room in almost all cites in VN for $10/nite. $250 for hotels. 'Eat street' and where locals dine and you have a food budget $5/day or $125. Use the open tour bus system and you will spend about $40 to get from HAN-HCMC. That is about $420 for rooms/food/basic transport for your 25 days. The rest of your budget: tours, shopping, incidentals is up to you. Using the overnight buses can save you about $10 each time. I recommend you use the overnight bus between HAN & Hue. Between Hoi An & Nha Trang. I suggest you hit DaLat before you go back to HCMC. Cool, green, pretty. But, you can go overnight between NT & HCMC if you so choose. That saves another $10. Hope this helps with your planning.