Sounds like you are following Dr. Seiler. Fast Talk podcasts have had him as a guest on several episodes. They discuss this issue that there is no accurate way to assess VT1 without lactate testing. The best approximation he mentions is breathing solely through your nose. In fact he went as far as taping his mouth shut while on they trainer. He also mention using controlled deep belly breathing vs chest breathing as aiding in controlling breathing levels below VT1.

Lastly, no need to train right at VT1, no energy system magic that are being used there vs. middle of the zone.