@Stefania ************
if you’re on a passport that qualifies you for the 45 day visa free entry, then you’re fine.
Vietnam Immigration officers are notoriously uninterested in anything about you except whether or not you fulfil the most basic requirements (such as having a visa or qualifying for visa free entry). I’ve entered Vietnam a dozen times and I’ve never had an Immigration officer say a single word to me.
But of course it’s like any country: they can ask to see an onward/return ticket or proof of funds or hotel booking or something else if they decide to do so. It’s extremely unlikely that would happen but it’s not impossible.