Dear Tweeters,
Thanks to Steve Hampton for posting about the Purple Finch's vocalizations. That was a helpful reminder. 
These finches can indeed sound almost exactly like Cassin's Vireos. A week or so ago, a friend and I were birding at Bryson Road, near Darrington. We heard two different Purple Finches giving astonishingly vireo-like songs. To me, these songs sounded more like Red-eyed Vireo songs than Cassin's Vireo songs. My friend and I agreed that many a birder would simply have recorded "Cassin's Vireo, heard only," or some such thing, since these finches were often out of sight when we heard them.
Most of the time, when I've heard Purple Finches give vireo-like calls or songs, the finch has reverted to its more typical vocalizations, but the birds at Bryson Road stayed with the pseudo-vireo renditions for long periods of time. 
On a similar note (pun semi-intended), there is another sound-alike bird vocalization that comes up just about every spring. I have noticed this one especially at the Fir Island Game Range, AKA Wylie Slough. A couple of weeks before Swainson's Thrushes show up, the Song Sparrows in that area will often append a "whit" note to the end of their songs. This note can sound almost precisely the same as the soft "whit" of a Swainson's Thrush. I have been birding with people there that time of year when this has happened. A couple of times I've actually gone to the trouble of tracking down the "offending" Song Sparrow, to prove to an incredulous birder that their thrush was really a sparrow!
Yours truly,
Gary Bletsch