Can I apply for F-1 visa to the US before I leave the US for vacation back home?

Most F1 visas for the US last for 1 year more than the average degree duration. In my case, the visa was stamped for 5 years for a 4 year bachelor's degree.

If your visa has not expired then you need to make sure to get a travel signature on your I-20 from your school's Department of International Education which is valid for one year and allows multiple entries and exits from the United States. If you ensure the same, you are free to travel anywhere you like and enter and exit the United States up to your free will until that travel signature expires.

Travel Signature on an I-20 is a signature that is issued by an international student advisor, which acknowledges that you are a full-time student in F-1 status and that you are maintaining your status at your University. The signature appears on page 3 of the I-20.

If your visa has expired or is due to expire before you complete your education, then you must contact your Department of International Education and request for a visa extension with a US embassy.

You can choose a US embassy in wherever location most comfortable to you as there will obviously not be any inside the United States. I would suggest choosing your home country though. Unfortunately, I don't think there is a way for you to confirm in any way whether you can return back to the US but you can certainly start filling out your application and get an appointment date such that it meshes well with your travel back home.

Also, since you visa has already expired you are legally obligated to go back.

In theory, you could start the application a few days before going home, so you can go to your appointment as soon as you arrive in your home country. However, this may not work:

• You may need to pay fees for the application. Sometimes the fee must be paid to the local bank, which cannot be done from the US.
• If you apply for an entirely new visa, you may need to pay a SEVIS fee. The receipt (which you need for the application) is mailed to you, so it may be difficult to coordinate.
• If you apply for extension (you probably are) then there may be no interview so that you just send your documents to the consulate. However, often you are required to use local couriers, which do not have an office in the US.

I would still get started before I leave (you need to do things like get an I20 signature and get documents from your school anyway), and fill out as much of the paperwork as I can. But don't count on it saving you more than a day or two from a process that may take several weeks.