Gilles & Joseph, if you’re having issues with NewBlue Titler Pro, please start a help ticket at newbluefx.com. Some things to start with:
1. Make sure you have the most current version installed. Download from here and install only the OpenFX version:
https://www.newbluefx.com/downloads.html
2. Make sure you have the current video driver installed.
3. You can also try various things in Vegas: reduce the Ram Preview amount, change the preview setting to Preview (Auto), see what happens if you turn off GPU Acceleration (just to see if it makes a difference), etc…
Edward Troxel