I'm a little confused by the exact details in this post, but I think I get the general idea. I would try putting a double condition on the acct_id to test for the existence of the default value in the parameter or filter using a single record only. Something like this:
{VENDOR_SALES.ORDER_DATE} >= {?Pm-@From Date} and
{VENDOR_SALES.ORDER_DATE} <= {?Pm-@To Date} and
{VENDOR_SALES.UNIT_SOLD} > 0.00 and
{VENDOR_ACCOUNT.SUFFIX} = 0.00 and
{VENDOR_ACCOUNT.SUB_CATEGORY} = {?Pm-VENDOR_ACCOUNT.SUB_CATEGORY} and
({?Pm-VENDOR_ACCOUNT.ACCT_ID} = defalut value or
{VENDOR_ACCOUNT.ACCT_ID} = {?Pm-VENDOR_ACCOUNT.ACCT_ID})