"Of Rears, and Vices, I saw enough. Now, do not be suspecting me of a pun, I entreat"
Making fun of the Royal Navy - 1796-style!
Public DomainMaking fun of the Royal Navy - 1796-style! - Credit: James Gillray (1757-1815)

 Rear Admiral and Vice Admiral are ranks in the Royal Navy, so Miss Crawford is referring to Admirals she has known. She, herself, notes the possibility for double meaning in the phrase 'rears and vices', and it has been suggested that 'rears' is intended to bring to mind the Navy's reputation for homosexual activity. However, bearing in mind Jane Austen's tendency to set great store by female propriety, one can't help wondering whether she would have allowed one of her female characters (albeit one of the more risqué ones) to have even hinted at such a thing. Possibly, therefore, Miss Crawford is only suggesting the possibility of a pun on the word vices, as she has already indicated that she considers the  behaviour of admirals as a group to be somewhat lacking in propriety.