To prioritize certain distributor's product of your choice due to preference, similar SKU and ect, navigate System > Configuration > Gunimport (Sub Menu) to the button left of the page.
Now select the manufacturer of your choice...
Click on the "General Settings" tab and change the last field to your desired priority number (0 would be top priority and 10 would be last priority) and click on the "Save Config" blue button at the top right hand.
Now you will need to flush your cache.
SYSTEM > CACHE MANAGEMENT. Select Flush Cache Storage at the top right of the page
Reindex if necessary..
SYSTEM > INDEX MANAGEMENT. Select All, Submit next to Reindex Data at the top right of the page.