Redirecting Quote Cost Values
The field used to control the cost used in Estimated Gross Profit functionality is found on the Item record. So, on each item, you can set the Cost Estimate Type field to Purchase Price.
1) It is not possible to use a specific Vendor purchase price. If you must use that, then either paste the Vendor purchase price into the general purchase price, or use Vendor Defined Cost and paste the Vendor price in there.
2) As the setting is per item, it is not possible to have Quote use Purchase Price while Sales Orders use Item Defined Cost, as an example.