Question about using TTS via the prompt element in VXML

In VXML, if we're using TTS via the prompt element, there could be a
variety of vendors a user could use to perform TTS. The VXML 2.0 spec
does not provide a mechanism to specify which vendor/product to use. Is
there a recommended way to specify this? Are there plans to add this to
future VXML versions?

Thanks,
Roopa