There are many. One major difference is that HVM/Bend focus entirely on functional programming, whereas Vine supports both imperative and functional patterns, in a cohesive manner.
Well, there have been several iterations of HVM, each extremely different from the last.
IVM is architecturally similar to HVM-64 (which I was the lead developer of). The most major difference is how they handle IO. IVM uses its extrinsic system, where all side effects are mediated through an IO handle, which provides a number of useful properties, and is greatly simpler to implement / use. Interactions with side effects are small and low-cost, and can happen in parallel with the rest of the program.
HVM-64 had built-in net definitions that had side-effects when expanded, which was very messy to use in practice. HVM2 has a monadic IO interface, which requires stopping the whole program on every single IO call. (And also requires writing things monadically.)
Using extrinsics for IO handles in IVM creates a very nice API for IO in Vine; side-effect-ful functions simply take a mutable reference to the IO handle. It's also very easy to support multiple 'threads' of parallel IO effects – simply duplicate the IO handles.
