Can I apply for a Schengen visa while holding a DTV visa in Thailand?

What is the country of entry you will use? You need to apply through there system, VFS have nothing to do with the actual visa so each country may have slightly different requirements when applying but you must enter through the country of application and then it is approved or denied by THAT country. VFS just processes the paperwork on behalf of the country you make application do but don’t make the decision.

Also be aware the UK isnt in Schengen, so you can’t visit the UK unless you also have a UK visa, and if you do that and your return flight is from a Schengen. Putney you would require a multiple entry Schengen visa.

Do you mean as a non-Thai? Well the DTV is basically a tourist visa - do you have to have residency to apply for Schengen in Thailand as a non-Thai? You have to be "Living in Thailand" - give it a go - they can only say no.

"To apply for a Schengen visa in Thailand go to the VFS Global application centre in Bangkok. You can only submit an application there if you live in Thailand. If you live somewhere else, go to the page Applying for a Schengen visa and select the country where you live to find out where to submit your application."