Von Neuman machines are even less likely considering they would still have to remain fully functional over a truly geologic timescale in order to be in any way functional. If you had technology that robust, it wouldn't need to be self-replicating because it would basically last forever. It's a Crazy Eddie concept that otherwise serves no practical purpose.
It's mundanely obvious that interstellar travel is perfectly feasible for an unmanned probe that can expect to be discovered intact after a billion years or so by an alien race that didn't exist when it was launched. Travel or transport LIVING BEINGS just isn't going to happen, though, without a space fold or something similar. Or -- as Gary mentioned -- if a civilization evolved near the galactic core or in a dense cluster of stars, in which case interstellar travel only takes slightly longer than interplanetary travel.
Still. greater numbers means two things: You still have more machines out there after falling prey to attrition through disaster, wear and tear, etc. Also more machines means covering the distance at sub-light more quickly, spreading like a virus,
But what good does this do us? Okay, so our legacy is an ever-growing set of self-replicating machines, spreading throughout the universe. Cool. What is the practical value, if it's going to take millions of years to amount to anything useful to living humans?