I interviewed for a VAP at an elite SLAC last year and was told flat out by many people that there was no room for negotiation on salary, but the salary was ~$60K, so I was okay with that. Unfortunately, they gave the position to another applicant who they thought was a better fit. :(
The school where I'm currently a VAP also said, albeit less forcefully, that their salary was not negotiable. (Too bad it was also much lower than the other place.) However, in a principled way, I pointed out how big of a difference the pay was with my then-current postdoc position and also pointed out my good teaching experience and credentials, so they raised the salary by $3K.
It usually doesn't hurt to ask in a nice way, but I think the key is the principled approach that you are taking (i.e., using an external source is a good start). I also think it is important to focus on your interests, not your positions. So, along with the other posters, I suggest that if there's not as much flexibility on salary, you seek other items like research space / good students to work with / conference travel / etc. that they might be more amenable to giving you.
Finally, I highly recommend the negotiation book "Getting to Yes", which is where I got some of these ideas. I've obviously used them in academia, but my partner and I also used them when we bought our house, and truth be told, I use them when negotiating with my partner all the time!
Good luck, OP!