Startup Co. ... I am US citizen, Co-founder is not?

There are options. A thorough evaluation of the facts can help determine which alternative will work best. Consult an immigration attorney.

For how many days can I take vacation to India in OPT period?

OPT allows 90 days (or 120 if extension granted) of unemployment. You can be employed and take vacation without the days being counted towards the...

Your case involves several issues. Best to seek out an immigration attorney and guide you through the process.

New reentry permit

You need to work with an immigration attorney for help with filling out CIS forms.

When exactly do employment verification letters become relevant in an employment based G.C application?

Best to work with your employer's attorney for curiosity kind of questions. Generally speaking, letters or other similar evidence/documentation...

Can trump impact a pending I-485 Eb2 ?

I will answer your question with an optimistic no but no one can predict what the new administration's immigration policies will look like and how...

I would like to know if it is possible to change the b2 visa to F1 while im inside the states or not ?

You referring to a Change of Status from B2 to F1. It is possible but is a highly technical process. Best to work with an attorney.

After approval of PERM, can I continue to work on H1-B or live legally in the U.S. if the decision on I-140 in pending ?

It is possible. Your employer's immigration attorney should be able to provide a strategy to accomplish this goal.

Can a E-2 investor visa holder in a startup with 1-2 U.S. employees apply for an E-2 employee or is the company size too small?

It is possible. Will require clearly articulated need for the special skill + evidence of possession of the special skill.

ABOUT APPLYING B1/B2

Your location is listed as Campbell, CA. Nevertheless, you can reapply but will have to demonstrate strong connections and intent to return to...