I'm sure that's true, but it still doesn't solve your employment issues, and you are just hurting wages for all existing plumbers. There are only about half a million plumbers in the US [1]. Even if you managed to triple the number, you're not making much of a dent in overall employment.
Sure, my point was just that if vocational jobs are naturally X% of the US economy, you're (largely) not 'creating jobs' just by making it easier for people to get trained for those vocations, you're just creating more competition within that sector and driving down wages of existing workers in the process.
That said, it might still be a worthwhile policy to pursue, since vocational education has been neglected for decades in favor of 'college for all', which IMO is misguided.
Vocational training is certainly part of your toolkit to increase jobs prospects for unemployed underemployed people who are willing to work, but it's not the whole toolkit. It helps, but you need more than that.
If there were twice the plumbers, the rate per hour would not be halved. There's demand that can't afford these services at the current rate.