Yosemite: Change keyboard layout of an input source My MBP has an AZERTY keyboard (a French keyboard, more precisely). I usually type English, French and Vietnamese. For the latter I use the built-in Vietnamese Telex input source. Since I'm used to type Vietnamese on QWERTY keyboards, I want to change the layout of this input source to QWERTY as well, but I didn't figure out how to do that.

Thank you in advance for any suggestions.
Just switch your keyboard to the US layout before you switch to Telex.  It takes the layout of the last keyboard used.