Traveling from US ->Hanoi Vietnam -> Phuket Thaniland -> Seoul Korea? cheaper to buy tickets in US?

I found a deal on Cathay airline that does Hong Kong + 2, 3, or 4 cities between $1099 and $1700 plus taxes. But I'm not really into going to HK this summer. Plus there are so much surcharges for staying over 21 days and adding on Hanoi that's not on the list of cities offered. I'm planning to go to Hanoi, to Phuket, to Seoul. On Kayak.com I found tickets for the entire trip for about $2100. Is it cheaper to buy tickets while I am in the country? or should I book the tickets in the U.S. before I go?