Hi, I am wanting to show certain vendors current accounts payable balance on the vendor form. ( I'm aware you can see this on the form anyway under the balance information - so for these I want to see this value in two places). Is there a way to calculate this and show this value in a custom field.

Yes if you make a summary saved search that show the correct value you are looking for you can use that saved search to source a custom field Here's a good article on it https://www.sikich.com/insight/how-to-create-a-netsuite-custom-field-from-saved-search-calculated-data/