Is there a European standard for example for what could be sold as vanilla extracts? The only definition of concentration and ingredients appears to be in US Code 21 CFR 169.175.
There's not really a European equivalent for the FDA's definition of extract (by ethanol percentage). Instead, the EU specifies what constitutes natural vs 'vanilla flavouring' (euphemism for 'artificial').
See the EU's white paper on it here (notably page 15): AUTHENTICITY OF VANILLA AND VANILLA EXTRACTS
Also, in plainer speak, info on vanilla industry standards between the US and EU here.
Additional reading: the EU directive ISO 9235 specifies what can be listed as artificial/natural and covers vanilla/vanillin.