Wanted to use multiple Lookup in FlowField

Hi Friends.

I have G/L Entry Table which has "Source Type" as 'Customer'/'Vendor' and "Source No" contains Customer Code or Vendor Code based on Source Type. Now I have added one more field called "Party Name" which is Text (30). Using this field I changed FieldClass = FlowField and put CalcFormula as "Lookup(Customer.Name WHERE (No.=FIELD(Source No.)))". This shows me Names from Customer table in above field if Source Type is "Customer" and blank if Source Type is "Vendor".

Now I wanted some other way (might be using code) which will bring Vendor's Name from Vendor Master. ](*,)

In short, the way by which we can define Lookup thru design mode can be coded in table. :?:

Thx in Advance.