I live in Saigon. The housing can vary from $300 a month for a room, or $400 a month for a furnished apartment in the suburbs, on up to $1000 a month or more for a nice place downtown. Internet I pay $13 a month for fast enough to stream sports and tv online. Mobile I pay about $5 a month for 450 mb data and texts. Electricity is $20 a month without A/C, I only prefer fans. With A/C will get you up to maybe 60 or 75 a month. Food budget can be cheap if you want to, around 5 bucks a day, on up for eating out a lot or whatnot. Consider the visa cost where you go and if you have to exit the country or can stay the whole three months. Uber, taxis and walking is all very easy in Vietnam. Renting a motorbike is about $50 a month, but please don't do it unless you are very careful and learn the rules (which many tourists don't realize exist).