Hi @Pedro87
This topic has been covered frequently on the forum and elsewhere. For example, this blog is very extensive on date handling 
To your case, you can approach it from multiple directions. One would be convert it to a date instance, and then convert it to your desired format.
Another one would to just directly extract the year, month plus and join it together (through Column Expression, String Manipulation)
join(substr(column("FEC_VENCIMIENTO"),8 ,2),"-",substr(column("FEC_VENCIMIENTO"),5 ,2),"-",substr(column("FEC_VENCIMIENTO"),0 ,4))
etc.