Combined/nested VTML for TTS?

Hi-

I haven't been able to find an answer to this question-  Can vtml tags be combined within other vtml tags? For example, I'd like to emphasize a word by changing pitch (higher) and speed (slower).

I tried to achieve this with the following: If you <vtml_pitch value="125"><vtml_speed value="85">DO </vtml_speed></vtml_pitch>need to add something, ...

However, I'm not noticing a speed difference.

Any suggestions? Thanks in advance!