Braintree's transparent redirect works beautifully, I don't have to pass any credit card info through my servers, and I'd like to keep it this way. My question is what is the preferred method to allow returning customers to use vaulted credit card/billing information? Credit card token is a protected field, so it cannot be submitted by the customer via an option field. Instead, I need to specify credit card token before generating the transaction data field. The problem with this is twofold, 1. handling disabled javascript if I were to attempt some AJAX and 2. forcing a returning user through a separate page so they can choose their credit card/billing info is almost as much hassle as re-entering the info itself.
You're right that using credit card tokens with transparent redirect is slightly difficult to deal with using the current Braintree API. 
However if you already have the users credit card information stored in the vault, you can use a server to server request since you won't have to capture any sensitive information. A simple HTML select for the credit card token field would work, and your HTML form would post to your own sever.
To make this solution even more comprehensive, you can have the tr_data field included, if the user wants to enter a new card you can submit the form to Braintree as a TR request.
If you have any more questions or want to work through this code together email Braintree support: support@getbraintree.com 
I'm a developer at Braintree and would be happy to help you with any more technical questions.
Braintree python transparent redirect with vault option