IF you are using "VM custom Field- Stockable Variant" plugin to generate "custom fields" then there is no way to make them appears as "radio" buttons. They appear inside a dropdown box. This is how Virtuemart was developed.
But there is an another custom field type to let you create "custom fields" with "radio" button selection, i can't remember right now which one was that, but try the "Integer".
I would recommend you to use the dropdown though, this is the better way and most common in well-known ecommerce websites.
Use, Custom Field Type: String (if you don't care about the stock of attributes)
Use, Custom Field, Stockable Variant Plygin: (if you want to stock control of attributes)
Beside those 2, the others are there just in case, they are not practically useful (and of course there is one more which can be used with "image" icon. It is also a "radio" button, as you might know, you can't put an graphic/icon inside the dropdown selection box. 2 days ago, we released a tutorial about how to add "image type custom field" to virtuemart. Take a look at it in case you might need it now or in the future:
www.flexiblewebdesign.com/blog/virtuemart-image-type-custom-field-tutorial-with-screenshots/
Regards.