flying from Kuala Lumpur to Ho Chi Minh

I have two months in SE Asia this June and July. I will arrive in Bangkok June 4 and make my way down through the Thai beaches and islands to Malaysia and Singapore, then fly from Kuala Lumpur to Ho Chi Minh, up to Hanoi and then back through Cambodia/Laos and return home via Bangkok on August 9. My question has two parts:

1. I see $80US flights one way from KL to HCM, which sounds like a great price to me. Since I don't know exactly when I will get to KL, can I expect to see the same price if I book only 24 hours in advance, or do the prices go up significantly if you do it this way, like they do in the US?

2. If that $80US price is going to go up significantly, I will book now. But for what date? I'm thinking 30 days for Thailand/Malaysia/Singapore, and 30 days for Vietnam/Laos/Cambodia, with a few days at the end for Bangkok. What is your opinion on this breakdown, and will that $80 ticket go up in the coming months?