A lot of these are a mix of genres, which is why people tend to say "you can't really call something one genre" or "there's no one specific way to define a genre of a band." What I can say, though, is that visual-kei isn't really a genre even if there are typical things that really say "this sounds like a vkei band." Our very own Zess wrote a very nice post in another thread discussing how vkei isn't really a genre, nor a scene, nor a movement, while still actually being all three at the very same time. What the fuck, Japan.
The Piass, for example, had several genre changes. They started out as, and for the most part remained, a punk rock band. They had some thrash metal in there as well, and their 2nd album had some happy / pop punk mixed with melodic rock that was en vouge from 1995~1999 (while still having very aggressive guitar work that Takayuki is associated with). Compare "Never Say Die" with "Over Again," and you'll see what I mean. Their third era changed genres again: here they got a bit more -core , slightly crusty, and even a bit nu metal here and there but still had a "punk" feel to them. Today, who knows what they sound like. They've added a 2nd guitarist, changed vocalists for the 3rd time, and haven't released anything in almost 6 years. For the sake of simplicity, it's just easier to say "The Piass were hardcore punk and have been around for forever" even if that is a broad, sweeping description of their discography which, compared to many other vkei bands, was very volatile.
From the bit of Diru I listen to, they were pretty typical to what was going on around 1998--they were just more creative about doing it, and since they're worshipped today they're often credited for inventing a lot of typical visual-kei styles (which, from an objective standpoint, is fairly accurate for their post-Gauze work). From the time period I like most, they were what was more or less pioneered by Kuroyume. Actually, a lot of late 90s bands fall under this genre which doesn't really have a name--it's a mix of post-punk, gothic rock, new wave, with black metal influence sprinkled on top (the manic laughter, "outbursts of emotion" a la Madeth Gray'll, frantic mumbling, bizarre gagging noises, pata-pata-pata drum structures, etc). Kuroyume started this with their first demo, then their first mini-album, but moved on from this with later releases. This style was more solidified with bands like La:Sadie's, Madeth Gray'll (who upped the gothic influence) and Aliene Ma'rriage (who added more an industrial spin to it), with other groups making their own variations like Due'le Quartz (Miyabi was awfully progressive and technical in his compositions) and DAS:VASSER (who seemed to be more into adding punk influences than adding harpsichord and pipe organ).
See how this is hard to really pin-point?