Help me to not screw up a potential job offer

I'd follow Rokusan's advice AND tell Employer B that you need to know very soon.

Play it safe. Accept Employer A's offer, hopefully with a start date that will allow a window of time for B to make a decision. If B hires you, then gracefully bow out of the job with A. Employer A won't like it, but you will have a job and that's what's most important here. /I know the ethics of that are a bit iffy, but having been through the same situation in which I did back out of a job offer, I'm glad I followed my own advice. Especially glad since my Employer A was Circuit City!

from an employer's point of view, the longer it takes for someone to make a decision, the less likely they are to actually take the job (or show up, even if they accept the offer). most people that really want a job will take it within a day and i usually ask (but not require) that candidates respond the next day after an offer. if a candidate went on vacation for two weeks without telling me i would assume they were a) not interested in the position and b) neither courteous nor responsible and would re-evaluate accordingly. lying is kind of your best option to keep your options open, although it is lame and would definitely burn bridges with employer A if you were looking for a job again in the future.

Ask employer A for time to think about it; a week should be the maximum you can ask for. Then accept the job, but delay the start date for as long as reasonably possible -- 3 weeks shouldn't be too much to ask for. In the meantime, mention to employer B that you've been offered a position, but you'd prefer to go with them so if they can speed it up you'd really appreciate it. If they offer you a position before you start at job A, tell job A that something came up. If employer B can't get their act together in time, it's too bad for you. It's a little sleazy, but it combines low risk with the best outcome for you. Another, even more sleazy choice: Even if employer B offers you the position after you start at employer A, accept it and quit job A with minimal notice; I don't know if this is legal everywhere, it's not a nice thing to do anywhere. I have seen someone quit after 1 week for his "dream job" elsewhere.

I don't know about other states, but in Texas, if Employer A is also a public school and you are a professional employee/teacher, you will have to sign a contract upon employment. You have until 45 days before the first day of instruction to change your mind. So if your Employer A is also a public school, accepting that job and then quitting to take the Employer B job may not be an option. They can withhold your teaching certificate if you breach the contract and that would restrict your employment for the rest of that school year. You can ask to be released from the contract but that decision is up to the district. Check the employment laws in your state if you are a teacher.

Don't lie and say you're on vacation for two weeks. If you're in the middle of a job hunt and suddenly disappear, it's likely they won't wait and will just go down their list. The ratio of applicants to jobs these days increases this possibility. At worst if you must lie, you might say you're out of town for the weekend, buying yourself until Monday. At the same time, inquire with company B as to your chances and timeframe. Also, it's "bated breath". Pet peeve.

Even if you got the offer from B, God knows how long it would take. A data point: I was selected to be hired by a university once, and despite everything the increasingly-enraged hiring manager could do, I was not cleared to start for four months.

Play it safe. Accept Employer A's offer, hopefully with a start date that will allow a window of time for B to make a decision. If B hires you, then gracefully bow out of the job with A. Employer A won't like it, but you will have a job and that's what's most important here. I agree - a (guilty-feeling) friend recently did this and donated his one month earnings from job A back to them, which they thought was very nice.

(re: 2nd paragraph, it should read "you have been offered a position elsewhere but you would prefer to work with Job B" and what the timeline is. After posting I saw I had ambiguous wording, and I apologise)