Hey there,
You can manage your form fields from VM backend. Goto VirtueMart/Admin/Manage User Fields. Over there you can edit the fields, create a new ones, order them, inform in each form you want to show them, and others. Besides that you can use the delimiter to separate fields in groups. Just create the ===Fieldset delimiter=== field, that won't be realy a field, but it will be really just a divider/delimiter. So, every field that is ordered below it in the Manage User Fields, will get into the "Group" until find the next delimiter field. Really cool.
Regarding the HTML, generaly, it is in PHP file that later will render/generate a HTML page, so normaly look at the address bar in your browser when you're on te form and see for example shop.registration, it will be the shop.registration.php file. So, edit it. Sometimes into this PHP, it calls another PHP files that shows at the same page, so you need to keep your eyes on it.
I hope that helps a little.