[VM] vm-pipe-messages-to-command messes up on virtual folders

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vm-pipe-messages-to-command seems to mess up the list of marked
messages and the location of the current message after running when
used in a *virtual* folder (it works fine in normal folders).

Specifically, if I have marked several messages in a virtual folder
and run next-command-uses-marks before calling a routine that uses
vm-pipe-messages-to-command, then:

1. Before vm-pipe-messages-to-command is called, the command
   'vm-marked-messages' seems to return the correct list of marked
   messages. 

2. However, after vm-pipe-messages-to-command is run, the list seems
   to be set to nil. This *only* occurs in *virtual* folders and not
   in normal folders.

Similarly, if the routine is called on a single message (without using
marks) then:

1. Before vm-pipe-messages-to-command is run, the location of the
   current message correctly corresponds to the message selected

2. However, after vm-pipe-messages-to-command is run, the location of
   the current message is set to some other message (sometimes a few
   messages later, sometimes a few messages earlier -- not sure of the
   pattern)

The bottom line is that when in *virtual* folders,
vm-pipe-messages-to-command somehow seems to mess with the notion of
the current message and of the list of marked messages.

Any thoughts on what might be going wrong here?

Thanks