Valve would probably not want people to use this, and it can be abused to give the modder advantages in the game.
Until Valve reimplements the -enable_addons launch option, it will not be 100% safe.
I would say that client-side modding was only "VAC safe" when they implemented the -enable_addons launch option which had a whitelist that specified what was allowed to be changed (announcers was one of these).
Honestly, since no one knows how VAC actually works, any mod that doesn't use -enable_addons is technically not VAC safe (since Valve does not explicitly state that it is allowed).
The new method I would not consider "VAC safe" because it modifies the file "gameinfo.gi" (a single line to what is a text file) and it loads an additional VPK before loading the standard VPKs (which I think is very similar to the old override_vpk).
America
Sorry I don't know since I've never used it
I've recently added some megakills from people who have asked me to but no one who makes kancolle ones has so far.
You aren't editing any existing files so you should be safe. In source 1, the -enable_addons option was created to allow sound mods such as this to be used. On Source 2, the -addon option method of modding is not officially supported, but you're not directly editing existing files so I don't think its a VACable offense.
I'd download GCFScape and open the pak01_dir.vpk in your dota folder and try to find the sounds you want to replace. Then create your own sound file and put it in your dota folder in the same directory locatio n as in the vpk file with the same filename.
I have considered making more/adding other's megakills into the pack but with Reborn coming soon it will be hard to make it customizable like on the site since the script file must now be compiled.