For obvious reasons, I want to access my computer's WAMP server from other devices (a smartphone and a tablet) in the same WLAN.
The httpd.conf part is okay, but I can't access my webapp unless I deactivate this Windows Vista's firewall.
Is there a secure way to enable only my devices to get in, without having to deactivate my firewall?
A firewall is pretty much never "all or nothing"; you can allow specific addresses, ports, programs. Use wf.msc to for configuring the Windows Firewall in Vista and later.
For example, you can add a rule allowing connections to httpd.exe, and/or to port 80, and/or addresses from the same subnet (under "Scope" / "Remote addresses").
Vista and above has this notion of "network location"s for this very purpose. The idea is that you tell it you are at a home and it will allow other devices with an IP address on your local network to talk to your machine.
Follow the instructions here to set/change a network location for the firewall.
If you did not allow Apache to operate on either network locations, you can change it by following the steps here.
You can of course add advanced rules to Windows Firewall to allow certain IPs/ranges etc (particularly using netsh advfirewall firewall for command line or via the "Advanced Settings" option in the Windows Firewall control panel applet which is a short-cut to something someone else pointed you to, "wf.msc"), but I believe that is beyond the scope/interest of your question.
Though these are Windows 7 links, it didn't really change much since Vista.
