Lisa Barlow and 2 companies she's associated with -- including her tequila brand -- are facing a lawsuit that alleges more than $400k was loaned out ... but never paid back.

The 'Real Housewives of Salt Lake City' star, Vida Tequila and Luxe Marketing are all named defendants in a lawsuit brought by a guy named Bart Carlson -- who claims he's a longtime personal friend of Lisa -- over loans he says he made to her starting over a decade ago.

The way Carlson tells it ... Lisa came to him in 2010 and told him she and her businesses were experiencing financial hardship, and she needed lotsa cash to cover expenses.

Carlson claims he agreed to give Lisa the money -- but didn't specify a time period for repayment because of his longtime friendship with Barlow.

Over the next several years -- beginning in May 2010 through February 2018, he says -- Carlson alleges he loaned Barlow, Vida and Luxe a total of $410,842.36 -- money he says he's asked for several times over the last few years ... without any repayment.

He says every time he's asked for the money, Barlow has allegedly responded by basically saying there's no way for her or her companies to pay him back ... and, now he wants some help from a judge to get what he claims he's owed.

BTW ... fans of the show know just how much Barlow mentions her tequila brand -- so, it's interesting to hear Carlson claim the company's been in financial trouble for years.