There is definitely a difference in the quality of vanilla beans & if you get ‘good’ ones, I can see where the vodka quality wouldn’t matter much, if at all. The vanilla definitely smells like vodka for a good while as the beans steep ... finally, you get that luscious vanilla smell, but it’s a bit of a wait.
One article I saw suggested making vanilla a year in advance ... always keep a steeping batch on hand. What I have now was started December a year ago. What I gifted last year, since I decided to do vanilla in November, was a nice bottle with beans/vodka in it & a tag as to when it would be ready (first available date - about 8 weeks) but also saying that the longer it steeps (like 6 months to a year), the better it will be.
For something like your lemon verbena liqueur, I would think the vodka quality ‘would’ matter more. Leaves vs beans ... the leaves wouldn’t be as intense I would think. Anyway, I’m glad it turned out for you! I grew lemon verbena many years ago in my herb garden and loved it (made tea out of it, if I remember correctly) .... might have to grow it next year (I have a spot) & try some liqueur. :-)
If you have a patch where you can put the lemon verbena, I say go for it. I have mine in a large pot and it spreads its shoots out and drapes all over. It’s lovely. It smells divine every time you walk past it, and when you harvest it and turn it into liqueur, it’s like liquid gold. It’s so beautiful. I found one recipe online but it has too much sugar for my tastes. I halved it and it was still plenty sweet.
https://wordsandherbs.wordpress.com/2012/07/22/lemon-verbena-liqueur/