The gold plastic on Vuldigus should be fine AFAIK (I've not heard any reports of it having problems). Gold plastic itself isn't necessarily poor - it's the quality of plastic. HasTak have notoriously used cheap plastic for their golds as well as the browns on Transmetal Megatron. Besides, IIRC it's not the same shade of gold as those with GPS. I only own the RiD Ruination version which has grey plastic, but like I said, I'm not aware of anyone who's experienced GPS w/ Vuldigus, so AFAIK it doesn't suffer from it. If you look up Gold Plastic Syndrome it has a list of all TF toys known to suffer from brittle plastic.

Sonokong Transformers are perfectly legit - they're not KOs and the toys are in no way physically inferior to their Japanese counterparts. Sonokong is a South Korean company that used to hold the licence to distribute TFs in S. Korea (now it's Mocom AFAIK). They essentially repackage Takara toys in Korean language packaging, instructions, tech specs etc, but the toys are the same. They packaging quality isn't always as good as Takara's, and may not get the right tech specs for that toy (and sometimes the tech specs card is just missing), and often the stats vary wildly from the Japanese specs (it's as if they just roll some dice to come up with their Strength, Intelligence etc.)... but let's face it... unless you can read Korean it doesn't really matter, does it?

But the toys themselves are fully legit and of good quality.