No
The original VecVoice used the GI SPO256. The SpeakJet (in the AtariVox) has the same allophone set (and a few extras), so the microcontroller (in the AtariVox+) uses a lookup table to selects them in real time (during the serial data stream).
Correct
If you flash the interface with CTS not inverted, it will not work in Phrasalator (with hardware flow enable). I used this as a 'normal serial' test.
Check if the USB interface has the attached settings.
You'll need this utility - https://www.ftdichip.com/Support/Utilities/FT_Prog_v3.8.128.448 Installer.exe
Avox_USB.xml
I remember the USB interface failing on really long phrases in Phrasealator. But a conventional serial port interface (powered by an external PSU) worked fine.
AAMOI I've always found USB-serial interfaces not to be as reliable as actual serial ports.