They laughed. Apparently picking up Spanish from your Spaniard friend has some downsides.
As in South American backpackers find it funny/amusing this drunk Chinese guy in an Australian pub used the Castellano equivalent of "y'all" in a sentence.
They were ultimately nice about it, much better than the time my (English speaking) Canadian coworker tried to speak to some Parisians in school level Québécois and they scoffed and continued in English.
Well, the way it was explained was "vosotros sois" is casual and "ustedes son" is more respectful. Like addressing your acquaintances or friends vs addressing your bosses.
Voseo is a weird subject and so is the use of vosotros. In Peru where I'm from, 2nd person plural is only ustedes for both formal and informal but 2nd person singular, tu is informal and usted is formal. Other countries and regions have what you're saying plus a 3rd level of further familiarity.
To my ears, saying "vosotros sois" comes off like reading scripture or arcane royal decrees. I immediately thought of Pan's Labyrinth where the faun speaks like that and the human characters mostly do not.
Yeah damn I wish someone told me that 72 hours ago