I just purchased the sef component and the virtuemart extension.

I install it correctly, but i have a problem:

When I active the component, the home page of virtuemat show the first category , instead the page for select categorys.

When I turn off the componet, this page is showed correctly.

Hello,

make sure the menu item name that points to VM "homepage" does not conflict with the category name. (so that they are not same) That could be the reason.

You can test this by setting "prevent duplicates" parameter in JoomSEF to NO.
Also you can find out by checking the URL in URL Manager. If there exists more "non-SEF" version for 1 single SEF URL, then that could be the source of the problem. In such case, you need to reveal a reason why they are same and where they originate from - an adjustment can be made then.
You can also use the "activate" icon next to the "correct" URL to mart the preffered one.