Populate Date field (CurrentDate + 30) in table if blank

I have a "Print" button on a Form called "Invoices" that that has embedded macros on the click event I want to replace with VB code due to the limitation in Macro. Currently this Macro does the following

1. Opens and runs a query called "SubtotalUpdate"
2. Opens and runs a query called "InvoiceTotalAppend"
3. Opens and populates a Report called "Invoices" with the DueDate from "Invoices" table before printing.
This is what I want to add to the existing macro all converted to VB code
4. If the DueDate field is blank in the "Invoices" table I want the date saved in that blank field in the table as (Current date + 30 days) before executing option 3. The format of the date should be mm/dd/yyyy. Can I replace this macro with a VB code. Can you please help with the code on Click event. Thanks

Looks like you already have it done:

HERE