Vandersteen 1c vs. 1b...drastic improvement?

There is not "That" much difference in the two speakers. The 1C is a touch more refined and and a tad more extended sounding(although the original 1B was spec'd with full range.) The 1C is a good sounding speaker and in some set ups sounds a little more coherent than the 2Ce. The 2Ce Signature is a better sounding speaker than the 2Ce(more refined)I would not go with an older 2Ci. You have to be careful with some of the earlier 2Ce's. They actually had the older 2Ci tweeter do to sourcing problems according to Vandersteen. When the 2Ce first appeared, it had the older 2Ci tweeter. Check with Vandersteen using the serial numbers.
Anyway, to answer your question---go with the 1C. You should be able to find a good deal used. It is a nice, overall sounding speaker.
Vandersteen makes upgrades that give a definite performance boost. The upgrade from the 3 to the 3A and then the 3A Signature are substantial. I personally consider the 3A Signature the best buy in the Vandersteen line for absolute performance for the money. It is a MUCH improved speaker over the 3A.

The 1c's are a significant improvement from the 1B, redesigned tweeter to eliminate glare and make them sound more open. Looking at prices for the 1c used, versus the 2ce, save your money until you can afford the 2ce. I have the 1c's and they are great speakers, but the 2ce's are really a big step up (they are probably the best value in Vandersteen's line). The one caveat is the 1c's will work fine with 50-60 wpc amps, the 2ce's want 150-200 wpc.

The Vandersteen 1b gave me my first extended exposure to the Vandersteen approach, which I find uniquely appealing. Subsequently, I have spent a lot of time with the 2Ce and 2Ce Signature (which differ slightly in terms of overall refinement).

Because of aesthetic (visual size), placement, and bugetary constraints, I purchased a pair of Vandersteen 1C's (new), and I hear an even more significant difference beteen the 1C and 1b than between the 2Ce and 2Ce Signature. I know they changed the geometry of the drivers somewhat between the 1C and 1b. This may account for the somewhat increased coherency of the 1C. A more pronounced change for me is in the virtual elimination of the glare that was present with the 1b's that could excite my tinnitis. I don't get that with my 1C's, I think, due to the change from the 1b's soft-domed tweeter to the 1C's well-damped metal tweeter, which also seems to project with more detail and better dispersion.

The 2Ce's and, apparently, a few of the later 2Ci's also benefit from the use of what sounds like a similar metal tweeter, by the way.

Based on my own farily extensive listening experiences with the 1B vs. 1C, I'd say the differences are subtle at best. Most of the changes in Vandersteen speakers, regardless of the model number, involve incremental improvements, and as such you will not find really major differences in the evolution of a given model.

If I were buying used speakers, I'd be inclined to get the 1B's if they were substantially cheaper than 1C's. If your budget will stretch to $500-600, however, I'd suggest you look for a pair of pampered, low-mileage Vandy 2Ci's. They offer a lot more performance than the 1B's or 1C's. The newer 2Ce's have somewhat better performance than the 2Ci's (which have been out of production for about 10 years), but they tend to sell for quite a bit more than the 2Ci's.