$30 a day all-in — bed, food, local transport, and one paid activity — is achievable across Vietnam and Cambodia if you keep long-haul transport to buses and trains rather than flights, and don't stay in District 1-style tourist cores every night.
Days 1–7: Vietnam (Ho Chi Minh City to Da Lat)
Start in Ho Chi Minh City (3 nights, ~$20/day), then take a sleeper bus north to Da Lat (about $10, 7 hours) for cooler weather and a break from the heat (4 nights, ~$22/day, since guesthouses and food are cheaper outside the big city).
Days 8–14: Cambodia (Phnom Penh to Siem Reap)
Cross by bus from Vietnam into Cambodia — a single bus ticket including the border crossing runs $15–20 and takes most of a day. Spend 3 nights in Phnom Penh (~$25/day) before a final bus to Siem Reap (4 nights, ~$28/day) for Angkor Wat, where a one-day pass costs $37 — the single biggest line item in the whole trip, and worth every dollar.
Making the Angkor pass work in the budget
Buy the one-day pass rather than the three-day if this is a first visit and your schedule is tight; a sunrise-to-sunset single day at Angkor Wat, Bayon, and Ta Prohm is enough to leave satisfied without needing the pricier multi-day option.
Takeaway: Treat the Angkor Wat entry as a planned exception to the $30/day average, not a budget failure — build it in ahead of time so it doesn't feel like blowing the trip.