You should not attach blind bracket to pvc frame. If the frame does not have steel re-inforcing inside the frame, your screws will simply pull out and leave you with a nasty looking hole in your pvc frame. After all, it's just plastic!
I do know what I am talking about - I own a company called All Weather Windows and install all types of windows. Your blind company will probably try to place the brackets near to the corners of the window. This is where the weld is situated and it is quite likely that the steel re-inforcing does not run all the way up to the corner.
You would be much better advised to contact a proper professional blind manufacturer such as ACME Blinds in Cork who have a new system that I have seen recently that clips onto the window beads and does not involve the use of screws at all. Ask for Kieran O'Connor there - he should be able to help you.