H1-B Visa Transfer Under Premium Processing

Am working in ABC company and got H1B this year (ABC did L1 to H1 transfer). My new H1 is valid from this year Oct’13.

Now, I got an offer from XYZ company who is filing H1B transfer in [b]premium processing[/b].

Here are my questions:

1. How quickly can I get approval of H1B transfer petition?

2. XYZ is asking me to join as soon as they receive petition number (tentative joining date is Dec 23rd 2013). Is it safer to join before I get approval?

3. I have to take vacation (international travel) during Feb'14, I will have to go for Stamping. Will there be any issues since I will be working for XYZ only for one month period (and I can only get two pay stubs only).

Please let me know