Have a look at the vgscan man page:
This is designed to find LVM information on your hard drives and recreate your lvmtab. (You don't by any chance have lvmtab from the old system do you?)
How by the way did it "find" your VGs? Did you do new pvcreate/vgcreate after the reinstall? If so you may already have lost the information you could have found with vgscan.
I'd probably deactivate the VGs before running the vgscan. You might want to save the lvmtab you already have as lvmtab.orig before running the vgscan so you can at least get back to where you were. Also doing diff between that and the one created by vgscan will let you know quickly if it actually found anything for you. After the scan reactivating the VGs and doing a vgdisplay should let you know if it found what you needed.