When Shannon told Vincent to look for Walt, he lead her straight to Boone's grave.

His way of telling her, keep looking for Walt and you'll die?

The vision of Walt seemed to be telling her to shush--another warning.

Another thought, which may be a spoiler:

It would be too sad if Vincent was telling her WALT was also dead, and it's Walt's ghost warning her.

Now who's going to take care of Vincent? Seems to me they really need that dog!