I do not own Vocaloid, but I'll try my best ot see if I can be of any help
Firstly, this FAQ question in particular addresses an issue using Vocaloid5 Standalone and VSTi versions. It's a bit unclear to me, but it can be interpreted as do not have both running simultaneously, or, and I am unaware if it's an option, do not have both versions activated.
As a general, make sure you have the VSTi located for Studio One to be able to find it. You can do this in your settings to see which folders Studio One is searching for VSTis in. Normally. Studio One searches for VSTs in the VSTPlugins folders in Program Files if you are using Windows. If the .dll for V5 is not in the folder, you can look for it and either tell Studio One to search for VSTs in that specific folder, or you can move the .dll to the VSTPlugins folder.
Also make sure that you are only using the 64bit version of Studio One and that you are putting the .dll in the 64bit VSTPlugins folder (Your PC/Computer > Program Files > VSTPlugins). Vocaloid5 is only compatible on 64bit systems and software, so if you are using the 32bit version of Studio One then you hae to install theh 64bit version (it is labeled as Studio One x64)
Those are a few possible solutions I could think of, I hope they were of any help to you. I'm sure someone who actually owns V5 could be of more help though. But maybe try those things or look around Vocaloid5's FAQ.