I wouldn't trust a VanitySearch site that isn't made by me or WhyFy.
The issue is more about who creates the site rather than what is used to create it.
Unless of course you were provided with a
Part Private Key in which case there is no chance of interference. I have rolled a handful of vanity wallets for others using this method via @LoyceV's thread - it was for me to gain some experience at doing such things (and I always ensured LoyceV was paid as it was their thread).
If a website were to take your
part private key then there wouldn't be any issue surrounding any website going rogue.
Except how many people know how to split the private key in the first place? Or know that there is actually splitting involved but you're just taking two random PKs and combining them together?
Most of the procedures people use to generate a split vanity address involves such voodoo as generating a random PK somewhere and then combining them on bitaddress.org.
There is
one post that does explain what Bitsddress is doing, and I wrote it, but it has yet to be featured in anyone's software.