If your setup uses BungeeCord you only need to install it on either BungeeCord or the servers behind the bungee (whether they are Spigot / Sponge etc).
If you choose to use BungeeCord, note that you will not be able to see yourself blocking.
You must use one of the following versions to allow certain versions, 1.8.x means any 1.8 version like 1.8.8.
curl -o ViaVersion.jar http://myles.us/ViaVersion/latest.jar
For development builds you can find them on our build server, https://ci.viaversion.com/job/ViaVersion/
Check out our documentation on our config.yml here.
/viaversion - Displays the version and avaliable commands
ViaVersion for versions 1.9 and above on 1.8 has to simulate player ticking for them to eat, draw bows etc. This means our plugin looks like it's causing lag but it's actually calling Minecraft server methods. You can turn of nms-player-ticking in the config to use an alternate method which won't touch timings but may cause issues with anti-cheat.
If you believe the issue is a bug, and not a configuration issue report it here on GitHub.