Hello,
Thank you very much for choosing our theme and for contacting us.
In order to register a vendor, you will need to install a multivendor plugin. WoodMart theme supports these plugins:
Dokan
WCFM Marketplace
YITH Multi Vendor
WC Marketplace
WC Vendors
As for the logo:
Please navigate to Woodmart > Header builder http://prntscr.com/iyd2pe
Choose the proper header type (your current) http://prntscr.com/iyd333
Modify elements as per your needs: http://prntscr.com/iyd3h3
Take into consideration that there is also the mobile view of your header http://prntscr.com/iyd3v3 check all the elements there.
If you have any questions please feel free to contact us.
Best Regards