VT1~1mmol lactate(steady state), VT2~4mmol lactate(semisteady state ie MLSS). Its actually just a continuum since its just a shaky balance between lactate production and lactate conversion back to ATP and lactate being an indirect measure of fatigue inducing metabolites. So small differences in power output or unsteady pacing or terrain can easily throw this balance off.
VT2 or Z3 in a 3 zone model is the middle of zone 4 in a 7 zone(level) model as pointed out by Carbonfiberboy above.
Over at the trainerroad forums someone has done a great job building an excel file to calculate your zones in both models based on your FTP/LTHR
https://docs.google.com/spreadsheets...it?usp=sharing
If you're interested in more discussion
https://forum.trainerroad.com/t/pola...g-podcast/1766
https://forum.trainerroad.com/t/pola...lon-show/14690
And Sieler's outline of his philosophy
Intervals, Thresholds, and Long Slow Distance: the Role of Intensity and Duration in Endurance Training