Using views as a source for staging table?
As has been noted in the comments, without more details, it's hard to give detailed help.  That said, I an provide some general advice.

If you're considering using vendor-supplied views to extract data for your datamart, you're first question shouldn't be about performance. Your first question should be, "Do the views have all the data I need?" You've probably already considered this, but the data you need is your driving force.

Second - if the source database resides in a different SQL Server instance, your data is being accessed remotely, a view may be a good option. If you access the data through, for instance, a linked server, hitting the tables directly may result in having to pull all the table data over to the active server, and then performing the joins. By hitting a view, you ensure that the joins will be processed on the source server. Now, there's nothing saying that the vendor-supplied views are the best possible views for your needs, but a view may be the best solution ultimately.