Multiple vendor code as GST registration, accounting done in wrong vendor code

1 Yes , but have to change GSTN & Address of vendor in books

2. As in regard to Inward supply for which No return has been started yet (GSTR 2) no need to worry just change GSTn of vendor in books . Yes if suppose vendor is local and GSTn is shown of interstate vendor , then that case you have to Reverse the IGST input & add same value to CGST/SGST input in your subsequent 3B

3. Not really any implications just modify the entry in Books of account