Lisa Vanderpump Sent Restaurant Tab to Archenemy Kyle Richards' Table

Lisa Vanderpump and Kyle Richards are such bitter enemies they're always looking for new ways to stick to each other -- such as Lisa sending her restaurant tab to Kyle's table.

So, here's the deal ... Lisa and her husband, Ken Todd, hit up the ritzy Shu Restaurant Monday in L.A. ... where Kyle also was having dinner with a 'Housewives' executive producer.

We're told Lisa and Ken sat several tables over. Lisa never went to Kyle's table to say hello ... nor did Kyle pay a visit to Lisa's table -- for obvious reasons, we think. But, the EP, who also produces "Vanderpump Rules" and 'Overserved' -- did notice Lisa and said a quick hello.

Now, get this ... when Lisa and Ken got done with dinner, we're told Lisa instructed the server to send their $132 tab over to the EP because she has a playful relationship with the producer, and jokingly felt he owed her a dinner. After all, they've made hundreds of reality TV episodes together!

The server obliged but, we're told Kyle and the exec both refused to pick up the tab. Clearly, they didn't get Lisa's joke, but not to worry ... Lisa didn't stiff the restaurant.

She tells TMZ she left a cash tip for the server and an imprint of her credit card in the event the tab wasn't picked up.