what are some dictionaries about vocabulary of classical chinese?
I would really recommend you focus mainly on Chinese-Chinese resources if you want to learn Classical Chinese, since it minimizes the distortion of the concepts and you'll learn a lot more Mandarin Chinese simply by exposure and seeing connections between classical characters and modern phrases. 

As Mr. Su notes, I've found the most useful Classical Chinese resources to be the 康熙字典 for individual characters, and the 辭源 for two-character+ phrases/allusions.  These two sources have done a lot of pre-selection against the definitions in other classical dictionaries, such as 說文解字, so save you a lot of time.  To ease the burden of looking things up in 康熙字典, I strongly recommend the "康熙字典Pro" iPad app (which I use daily - can one say "手不釋iPad"?): 


you can use handwriting interface to input characters with non-obvious radicals
you can tap on unfamiliar characters in the definition (which is going to happen a lot!), pivot to their definitions, and then backtrack instantly to the parent definition
it includes pinyin pronunciations so you don't have to guess (as in paper versions) at the modern pronunciation from 反切


You're going to want every advantage you can get when tackling with this stuff, so don't shy away from new technology: if some enterprising Chinese company wants to make an iPad version of 辭源 I'd imagine there's a market for that, as well.

At present the only Chinese-English dictionary somewhat useful for Classical work is Mathews' Chinese-English dictionary.  However it is seriously out of date and I remember it contains many innacuracies.  The as-yet unpublished "A Student's Dictionary of Classical and Medieval Chinese" (http://www.amazon.com/Students-Dictionary-Classical-Medieval-Handbook/dp/9004284117/ref=sr_1_1?ie=UTF8&amp;qid=1418329348&amp;sr=8-1&amp;keywords=9789004284111) may be worth your consideration: given the pedigree of the authors I would expect it to be much more dependable from the philological and formal linguistic perspectives.  I'm not sure it makes sense to organize a dictionary for a non-spoken language like Classical Chinese according to anything other than radical and stroke, though - why organize according to Pinyin that's only meaningful for modern Mandarin Chinese?