like cafe

said, clothes sandwich is a nice option!! ^u^
it can protect them and at same time you organize better the space ^^
also, this is a little extra and not really necessar,y but you never know.. in case you have, keep a copy, a photo or pdf of purchase receipt of your vocaloid boxes (this is more for the customs at arrival, if they check your bag), and don't worry about anything, if they ask you what are those, just say the simple truth, pc music software

you could put the vocaloid boxes also in your backpack if you plan to use one ^^
on the plane you can bring almost anything, really: the security is there to check for sharp objects, explosives, drugs, .. anything else is ok ^^
The TSA "What Can I Bring?" page provides a comprehensive list of items that travelers can and cannot bring in carry-on and checked baggage. It includes guidelines on common items such as liquids, electronics, sporting equipment, and medical devices. The page also offers special instructions for...
www.tsa.gov
as written on the TSA website, objects like books need an additional screening, but that's simply because is how x-ray works, so they might do a second screening of your bag, or ask you to take out the boxes and do a screening of them separately, don't worry ^-^