Multiple Entries/Non-Consecutive Stay

Good afternoon. I will be entering Vietnam (Ho Chi Minh) and staying for two days and then traveling from Ho Chi Minh to Bali and onto Cambodia for a total of 18 days. I will then fly from Cambodia back into Vietnam (Saigon) and stay in Vietnam for 12 days and then fly from Ho Chi Minh back to the United States. Since I will be in Vietnam for a total of fewer than 30 days (14 total) but the days are non-consecutive and my total trip in SE Asia will 32 days, I am unsure whether I need the 1 month multiple entry or the 3 month multiple entry. I appreciate any help you can provide. Thank you!