The detailed information you've shared is very helpful, @shaun. I appreciate that. 

Let me provide you with some information on the migration process as well as steps on how to fix missing VAT and debtors'/creditors' opening balances in your QuickBooks Online.

Ensure that the multi-currency and VAT are enabled before importing the data, as these will impact the imported lists. Check out this post to learn how to handle your VAT import and record previous firm opening balances: Enter customer opening balances in QuickBooks Online.

Once confirmed and the issue persists, I'd recommend logging into your account using a private browser to verify if this is a browser-related problem. Sometimes, the browser is full of frequently accessed page resources, thus causing websites like QuickBooks Online to act weirdly. 

You can use the shortcut keys below:

• Ctrl + Shift + N (Google Chrome)
• Ctrl + Shift + P (Firefox)
• Control + Option + P (Safari)
• Ctrl + Shift + P (Microsoft Edge)

Once logged in, re-check if the VAT, debtors, and creditors opening balance are now visible in your account. If it's fixed, please return to your regular browser and press the CTRL+ Shift and Delete keys on the keyboard to clear the cache.

In addition, check out this hub to learn how to troubleshoot errors after conversion and create post-conversion checks to ensure that everything in your QuickBooks is working correctly: QuickBooks Desktop discontinuation support hub. 

Lastly, you can bookmark our QuickBooks Online Help Articles and Video Tutorials for your future use. These contain various topics on how to manage your business. 

Don't hesitate to get back on this thread if you're desperate for assistance with QuickBooks. I'll help you out. Take care always.