Lindsay Lohan partied like it was her birthday on Saturday -- because it was her mother's. According to the manager of Mio Posto in Hicksville, L.I., the 26-year-old "Liz & Dick" actress had "double vodka and club soda" during Dina Lohan's 12-person ... small but rowdy celebration.

Dina brushes this behavior off, and says its no big deal. Its this kind of thinking that encourages the habit:

"I'm certainly not making excuses for Lindsay's behavior. But she's a 26-year-old woman. Some people who want to quit drinking do it cold turkey and some do it gradually -- you want to keep a balance. I don't know what's a right way or a wrong way."