This ink may be the winner IF an ink labeled "sheening" won't cause a problem.
Smug, am I correct that you would say ink #7 shouldn't cause problems?
Yes, I'd say that, ...
... but that's me and my appetite for "risk" when it comes to the use of my fountain pens, which some seem to think amounts to an unhealthy and unbecoming disregard for objects (and/or brands with plenty of history and heritage).
Notwithstanding that Sailor is my favourite brand out of the Japanese 'Big Three', which each has over 100 years of history, here we're talking about a converter-filled fountain pen with a mass-produced, dark-coloured plastic body with a basic gold nib in the Fairy Tales Vega; what kind of problems could anyone foresee being caused by any commercial fountain pen ink? The ink can't stain the gripping section, the nib and feed are friction-fit and can be pulled out easily enough for a deep clean if necessary, and the converter can be fully disassembled should its inner walls require a light scrubbing if flushing doesn't clear all the colour out when switching inks. As with any other ink, it's a good idea not to let the ink dry inside the pen; but I can trust the cap on a Sailor Professional Gear Slim to seal very effectively when the pen is unused.
Sailor makes heaps of highly-saturated and sheeny inks, in its Shikiori and other product lines. I personally wouldn't worry about undue risk from using any of those, or Diamine Cult Pens Iridescinks (sheeny), in my Sailor Professional Gear (Slim) pens, when I'm happy enough to put iron-gall inks (e.g. KWZ Ink IG Turquoise, Hero 232) and particle inks (e.g. Sailor souboku, Platinum Carbon Black, Diamine Shimmering Seas) in them. I have an ultrasonic cleaner, and now finally some (industrial-strength) aqueous ammonia. (Clear ammonia is not something Joe Consumer can just buy in supermarket, pharmacies or building hardware stores in Australia.)
You probably already know whether Diamine Inkvert Festive Cheer would dry on the nib when pausing one's writing briefly, from its use in other pens in your operating environment. That's one thing I cannot take into account; Sydney weather is pretty mild, all things considered. Sailor's nibs and feeds aren't commonly prone to that kind of issue themselves.
As always: 1. Implicit in everything and every instance I write on FPN is the invitation for you to judge me as a peer in the community. I think it's only due respect to take each other's written word in online discussion seriously and apply critical judgment. 2. I do not presume to judge for you what is right, correct or valid. If I make a claim, or refute a statement in a thread, and link to references and other information in support, I beseech you to review and consider those, and judge for yourself. I may be wrong. My position or say-so carries no more weight than anyone else's here, and external parties can speak for themselves with what they have published. 3. I endeavour to be frank and truthful in what I write, show or otherwise present when I relate my first-hand experiences that are not independently verifiable. If it is something you can test for yourself and see the results, I entreat you to do so.