[G6Tanuki]

I always considered the original Vibroplex concept to be a horrible thing: It sent dots at a fixed rate [set by the springs] but dashes were left to be timed by the operator.

Fine for line-telegraphy, but on HF radio channels, where propagation conditions were continually changing, it made no sense. You might get a few minutes where the channel was good and you could do 20WPM but a few minutes later you could be in a deep fade which was only workable at 5WPM with loads of retries.
Being stuck with your dot-spring-setting at 12WPM meant you couldn't go-faster when the channel was good or slow-down-to-a-crawl when it faded.

QLF? Are you sending with your left foot??