Hi,
Completing a self assessment tax return does not trigger VAT registrations.
This is something you have to do separately from self assessment. The foreign self employment income is added to your gross self employment turnover in SA104F.
You then enter the VAT charged as an expense, which is set aginst the gross turnover, so that you arrive at the taxable net profit.
You would only need to declare in SA106 as well, when you have paid foreign tax and wish to claim a foreign tax credit:
Register for VAT, which advises of the action to take when your VAT taxable turnover is over £85k.
Thankyou.