Slovenia was not formed until approx 1990/91. What country would Franc & Ivan have been born in? Yugoslavia, Austria/Hungary, etc? Census records from 1920, 1930 would not show the country, Slovenia, as it didn't exist when these two immigrated to the US.
Can you provide an American pronounciation of last name? There are only a few people in the US census with last name of Vucanjk. Name was probably shortened or changed to something more "American" (change in spelling to reflect pronounciation).
Are you saying Franc/Frank married a woman named Lena & his brother, Ivan, married a woman named Lenka?
For your information, the latest census available in the US is 1930.
Can you be more specific than "were around 20 years old" and "immigrated...about 1920's"?
To do a proper search in the US census & other information contained in Ancestry.com [& other paid subscription services] one needs: full name, date of birth, place of birth. With "difficult" searches such as yours any information you have could prove helpful. At least you have a location in the US where they lived, that should help some once we figure out their ages & place of birth. But, the last name is going to be a problem. Any idea what kind of work these two did? Were they, perhaps, trained in a skill such as stone mason, brick layer, shoemaker, etc that they could have pursued in the US?
Would be glad to try to help you but you have to provide other information first.