We have

Invoices

and we have

Vendor Bills

And the built-in fields of NetSuite on

Invoices

have a

Est Gross Profit

field, but out of the box it doesn't seem to link transactions together (which seems reasonable?)

Netsuite does not calculate profit that way. The "Est. Gross Profit" field has a very important "Estimated" abbreviation in there. There is no NS functionality that will show you clearly in a field what the profit of a transaction is. I have done custom programming in the past to get that in situations where transactions were pretty much always linked (always Special Order) situation, but it's nothing native. And even that gets cloudy when you factor exeptions like credits and etc.