Hello there, VAZ.
I'll ensure to provide information about the Total Sales in the VAT return that doesn't match the Total Sales in Profit & Loss for the same period in QuickBooks Online (QBO).
In your case, the total sales in the VAT return will not always match because there are transactions that can appear in a data file that make it unlikely for the Profit and Loss reports to match.
Additionally, the total of the VAT return is specific for all the transactions that have ticked as taxable and have tax calculated. The total sales for the Profit and Loss include all the taxable and non-taxable transactions and include transactions that do not use items like the expense tab on a bill or a journal entry.
I'm attaching these articles for future reference in managing reports in QBO:
Tag me in the comment section below if you have a follow-up question about the mismatch in total sales in your report. I'm always around to assist you.
