I don't think the Vilani are corrupt, though some Solomani might see them that way. If asked why they're so corrupt, I personally feel the Vilani would ask why the Solomani are liars.
Yes, you're expected to bring a "gift" of a Naaru fruit to a bureaucrat when you're delivering paperwork to be processed or else the bureaucrat won't do anything with it for months, even years. If you offer him a Naaru fruit, it'll get done in the order he gets it (it gets put in the stack). If you bring him one of the more expensive Naaru fruits from Dagu, he'll get your paperwork done before the end of the day. If you get the Sylean-breed Naaru fruit, one regarded very highly for its sweet nectar and fragrance and difficulty to cultivate, he'll process your application immediately while you wait. There's even more expensive gifts you can give, but at that point there's customs on the limits of how expensive of a gift you can give a clerk, after that you start gifting the clerk's boss. Heck, there's probably a few stores or stands selling Naaru fruit near Vilani bureaucratic offices, though the more expensive ones you have to go downtown. (Naaru fruit doesn't actually exist OTU to my knowledge - this is just for purposes of illustration.)
As a Solomani, you might see this as insufferably corrupt. The Vilani would reply that everyone knows these things, it's a custom that's endured for three-thousand years. They'd ask why our "Amazoom-Egg" claims we can pay $10 more to get our order expedited to one-day delivery. Isn't that a bribe? And not only do they brazenly expect this money, when you pay it, the product might not even come on the next day despite the implied promise. Of course, when you actually read it in the legalese you discover it's somehow not binding, despite the fact Solomani society is proud of their "truth in advertising" laws, and you actually have no legal recourse if it doesn't show up on the same day. How is that any better than their system?
Another example might be haggling. Generally in the modern 21st century West, haggling is somehow seen as a sign of "those corrupt eastern cultures." Perhaps the Vilani haggle over many things at market. Yet, the Vilani might get this knowing smile and ask, "You claim haggling is terrible, yet in your culture you call it 'negotiation' and for large contracts and purchases it is normal. You Solomani do it to buy an automobile, a home, or when determining the price of large orders between corporations. Yet, because your merchants can lord over you in day-to-day purchases with set prices...you have no experience with negotiation when there are large sums of money involved, so you have great difficulty with it and negotiation is considered some specialist skill possessed by a select few and the wealthy instead of a life skill everyone should know."
The part where the Imperium changed a flag to colorless because one alien race could only see in infrared or something was just ludicrous. I don't remember which book that's in.
It's not ridiculous at all.
It's
officially colorless.
It's necessary to give the Imperium the legal means to render the Imperial Sunburst in whatever way is necessary for a sophont to see it and understand it to be the Imperial Sunburst. If you can only see in the infrared, x-rays, or use echolocation to "see"- the local Imperial presence can render the Sunburst in graduated heat-patterns, bas-relief, or whatever else is necessary for the local sophonts to recognize it;
without it, the colored image of Imperial sunburst in red looks like a blank piece of paper to the IR-visual types. Once the "official" display standard for IR visual types is determined, then that version of the Sunburst is official for that sophont species and has co-equal authority as Imperial stationery as the familiar red sunburst.
It's just a bit of cocktail party trivia in most of the Imperium. For humans and others who share a visual system reasonably like humans (eg; the overwhelming population that participates in the Imperium), nothing changes; the official color of the Imperial sunburst remains a certain shade of color on a certain background.