Looking for some general advice

Hi and welcome to the forum.

Let me try to answer your questions as best as I can....

1) Would it be possible to get a teaching position only for the summer (May-August). I know many places require at least a year contract, so I am wondering if there are any summer specific positions.

Most Intl. schools have summer programs primarily focused on ELT and STEM workshops (e.g. AIS Vietnam).

Reputed ELT centers have summer specific short-term programs. You need to visit their websites to check available opportunities. (e.g. an old ad from Apollo VN)

Even British Council VN has some old summer jobs on their website.

2) Should I apply for jobs before arrival? If not, are there many jobs available so I would not have difficulty finding one when I get there?

For jobs mentioned above, you can (should) apply as soon as these jobs are open ... preferably from outside of Vietnam.
The school will then invite you on a business visa which is what you will be needing to work legally in Vietnam for not more than 3 months.

I'd suggest you submit your resume and expression of interest to all those schools well in advance.

For regular ELT jobs it's better to look for a job while you're in Vietnam.
Not sure if that'd be a good option for the summer jobs. Opportunities are limited.

3) Housing? Due to the duration of my stay would it be better to just rent a hostel room for the 3 months? Are there many rooms of expats available short term?

If you can land a summer job with a reputed ELT or Intl. school, they will take care of your accommodation. ... or, will help you find one.
Yes, you can rent a private room (if not an apartment) for three months.

4) What can I expect with work load? I have experience teaching so I have a general understanding of what teaching entails.

Sorry, can't help you on that. Summer schools must be more fun and less rigid.

Wait for other ESL teachers on this forum to enlighten you on that.

Hope this info helps!
Cheers!