hi! been to vietnam 2x for business and once for vacation. P30000 for 2 people is more than enough if you do the tours yourself, sarap ng food and mura lang. meron pang for shopping.

airfare (cebu pac promo): 9 thou (allow P1650/head for the usual travel tax at terminal 3)
hotel: 1200/day on the average, but there are still cheaper ones; i've 2 good recommendations (anan2 hotel and mini-saigon hotel)
meals: ave per meal at sassa cafe in de tham, district 1 is P100-150/head
ave P200 at highly recommended nha hang ngon in pasteur
ave P50-100 in jollibee

around 11 stores in ho chi minh
P150 at pho 24
P20-30 for banh mi
P500/day for meals is more than enough and provides you with the variety and culinary adventure you'd enjoy, there are also a lot of 24hour convenience stores in the tourist area for quick snacks
taxi: estimate P350 from airport to a district 1 de tham hotel; then P100 hoteltto shopping places like ben than and saigon square 2 (where both northface original and class a copies are sold, kipling bags/wallets all copies, crumpler);
CAUTION; take only Vinasun or Mailinh taxi anywhere. all other taxis are colorum, they either overcharge you or rob you
tours: independence palace, notre dame cathedral, post office, opera house which you all can do walking from one site to another; si independence palace lang may bayad, P40 lang ata yun;
their unique theme park is near dong nai, can give you instructions to commute
cu chi tunnel, a lot of tourist shops offer this
from ho chi minh, you can have a connecting tour to cambodia esp siem reap where angkor
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but you need to free up 3 days for the combined land travel and tour
or take a local flight to da nang ($90 round trip) or nha trang. mura lang vietnam airlines and tour offices are accommodating and helpful. or take a countryside tour to can tho for only $20-25 overnight with simple accommodation and 2 meals