General Budget for Basics, For a Single Guy in Vietnam??

Food  : 220$
housing 250$ ( include electricity ,water, internet....)
entertainment 100$.

A  well-furnished 2-bedroom apartment would probably cost upwards of 400$ which includes TV, water, internet (hi-speed) and cleaning services.

If you are into street food, then you can limit your food expenses to about 200$ or even lower.

A good scooter would be upwards of 1000$, but you can find many decent 2nd hand ones for about 250$

If you plan to stay in hanoi, you would need atleast 1000$ a month to live comfortably provided you are not sharing the apartment with someone.

Thanks, that gives me a better idea of what to budget.   

Saigon
That's a bigger apartment and a lot of eating out, difficult to estimate.
Assuming finding/negotiating furnished apt in D1 or D7 through English-speaking broker, allow $1400+/month if you are doing foreign food, $1200 if Vietnamese food at quán ăn.

$2000 for a new Airblade 125cc, or $1500 used 2013 Airblade (first year 125cc).
But I believe to purchase and register a motorbike in your name you need a TRC (temporary resident card, maybe via your employer)