Van Buren was not even on the ballot in any slave state except Delaware--where he got 0.66% of the vote.
http://uselectionatlas.org/RESULTS/state.php?year=1848&fips=10&f=0&off=0&elect=0 If the election went into the House, this by itself would be enough to keep him from winning, since the slave states controlled half the House delegations.
Moreover, he was extremely weak in many of the free states as well--e.g., Pennsylvania, where he only got 3.06 percent of the vote,
http://uselectionatlas.org/RESULTS/state.php?year=1848&fips=42&f=0&off=0&elect=0 or New Jersey, where he only got 1.05 percent.
http://uselectionatlas.org/RESULTS/state.php?year=1848&off=0&elect=0&fips=34&f=0 He didn't do much better in Indiana with 5.30 percent.
http://uselectionatlas.org/RESULTS/state.php?year=1848&fips=18&f=0&off=0&elect=0
Moreover, he couldn't get a lot of antislavery Whigs to support him: Benjamin Wade and William Seward (not to mention Abraham Lincoln) all supported the Louisiana slaveholder Taylor instead.
In fact, militant antislavery men had good reason for viewing Van Buren with suspicion. As James Russell Lowell's "Birdofreedom Sawin" remarked:
I used to vote for Martin, but, I swan, I 'm clean disgusted,—
He aint the man thet I can say is fittin' to be trusted;
He ain't half autislav'ry 'nough, nor I ain't sure, ez some be,
He 'd go in for abolishin' the Deestrick o' Columby;
An', now I come to recollect, it kin' o' maks me sick 'z
A horse, to think o' ivut he wuz in eighteen thirty-six.
http://books.google.com/books?id=vD5BAQAAMAAJ&pg=PA205
In any event, there is just no way Van Buren is going to get elected as a Free Soiler in 1848, period. If at the last minute Cass is caught with the proverbial dead girl or live boy, many of his supporters will vote for Taylor (who after all did not have a record as a partisan Whig) rather than Van Buren, and others will stay home.