I have used this in the past :
https://store.webkul.com/Joomla-Virtuemart-Point-Of-Sale-POS-System.htmlIt worked, but needed a lot of custom coding to suit my needs. Almost all my products are stock controlled child products and this extension doesn't support that - it just displays every single product as it was a parent. No category filtering either, so in my case it simply loaded too many products and was too slow to be usable. And I don't even have that many products (around 1200).
I added a category filter and in my case I don't sell the exact same products online and "in store" so I managed to make it work OK - I needed it mostly for receipt printing, stock control of these particular products and reporting.
I eventually moved on to other solutions though (a totally different POS).
What about using a specific template only for POS but using your virtuemart shop directly ?
This need some development but is easy to maintain
This sounds like a clever idea in my opinion

. Guess you'd still need to "bypass" the regular checkout process though somehow, as we have to be able to checkout with just the click of a button and no customer data (generally not even e-mail, unless the customer is kind enough to give it to us

).
VM's checkout process can probably be modified for this purpose but we still want to allow normal checkout for online customers outside the POS I suppose

.
But it does sound like the easiest way to achieve this imo.