The state of Imladris during the War of the Ring

I doubt if anyone could lay siege to Rivendell during the War of the Ring.

First of all, who could have done it? Sauron was too busy trying to decimate Minas Tirith, the Elven enclaves in Mirkwood, and Erebor. Saruman was already overthrown.

Secondly, how? Sauron cannot send a large army to Rivendell: Gondor blocks the south passage, while Erebor and Dale blocks the northern one. Sure, he could send forces over the middle portion of the Anduin, but I'm sure the residents of Rivendell could handle them.

Saruman, like I said, is overthrown. His army is destroyed. Sure, he has a bunch of ruffians operating there, but sending them against Rivendell is like "a child threatening a mail-clad knight with a bow of string and green willow."