Visa transfer during vacation

I am the full time employee of company X and now I am on unpaid vacation 40 days. Whie I am on vacation, I have got another H1B job in Y company and Company Y has now sent my Visa application to the immigration. The Y company is asking me to join as early as possible because I am eligibe to work with them (Y company) as a full time employee because they (Y company) has filled for the H1B transfer.

I know that when I will join Y company I will still have 20 days in my hand as the remaing vacation period with X company. and If atall I resign the X company I will have to give notice in 15 days advance.
My questions are --
1- Can I join company Y as a full time emplyee while I am alrady fulltime employee of X company and I am on Vacation?
2 - Can I give resignation to X company when I am on vacation so that I can be considered the full time employee of Y company?
3- Is it adviceble to join company Y if I just have the receipt of    H1B filing by Company Y, or I should wait untill I get the confermation that my visa has been transferred.
4 - My pay slip that was used by Y company is the one that was issued to me on 4/10/2008, and not the one that was issued to me on 5/10/2008.(please note that they X company has given me the "Short pay" in last month the pay slip of which was issued on 5/10/2008)
So will my visa still be able to transferred in Y company or it will be denied because the Lower of Y company has submitted the pay slip that was issued to me on 4/10/2008 and before