Member ID Custom Field not displaying for Vendor front end

Hello,

Since you want to display a user field into an address ; I'm not sure that it will work perfectly by using the "address_template" system.
I suppose that you have override for your email so you can add the field after the display of the address.

So for HikaMarket it will be the same process ; creating a view override for the "ordermarket / show_block_address" to display the field value after the display of the billing address.
You will find the call of the "maxiFormat" function ; and you want to display your content just after.
Since you will have access to the "$this->order", you will get the user object and its custom fields.
And in the view you also have variable to detect if you're display the billing or the shipping address, so you can add the condition to only display it for the billing.

Regards,