I am on H1B Working for company A. I put vacation on company A and transferred my visa to company B. I was working with company B for one week and I don't like it. Can I switch back to company A. I got pay stub for one week work in company B. Is that a problem ?

No problem but not easy...
If you can explain why, and company A is willing to take you back.
You better have an experienced attorney to help you with this issue.