I can relate! When it comes to the audio tech side of things, I’ve been in that same awkward spot for years now, it feels like. 
I’m not sure how much help I’ll be, but I’ve semi-frequently experienced somewhat similar behavior, and I can share a few things from my experience that might be worth trying (either now or next time you encounter VE Pro weirdness):
- Have you tried resetting the SMC and the PRAM? This sometimes fixes VE Pro problems for me, though I confess I don’t fully understand why it helps in the context of VE Pro.
I would be wary of doing this on a regular basis (e.g., several times a week) … but if it bears fruit for you, I think it’s fairly safe to do it up to once/every couple months, only when necessary because you’re experiencing weirdness. In case you don’t already know how to go about this, here’s an article (I would take the “when & why” part of the article with a grain of salt in this context, as in my experience nothing it mentions has been relevant to VE Pro).
https://thenextweb.com/lifehacks/2017/06/14/how-when-why-to-reset-the-pram-smc-on-your-mac/
- My personal experience with Event Input is that it has never worked reliably with notation software. If this method is working well for you longterm, then great … but if you start to notice problems recurring semi-frequently, you might consider an alternative setup, such as:
2a) You might be able to use multiple MIDI ports, since Dorico loads the VST3 version of the VEP plugin. I’m fairly sure this works reliably, but I don’t have too much experience with this method because unfortunately, other important plugins in my setup prevent me from being able to make use of multiple MIDI ports.
2b) You could use multiple project instances to accomplish a similar end result as you get from the Event Input or multiple MIDI port approaches. There are aspects to this approach that aren’t ideal (I would prefer to have everything in one instance, just so I didn’t have to click between various tabs), but in my experience on my machine, it’s as rock solid as VE Pro ever gets connected to Dorico. For anyone going this route, a friendly bit of advice I wish I’d known long ago: Don’t only save your “Server Project” but in addition save each individual instance separately. In other words, don’t only save the .vesp file, but also save all the .vep files that are housed within your .vesp file. I’ve learned this the hard way over and over: sometimes the Server Project (.vesp) files can get corrupted suddenly and without warning, such that they crash VEP before they even load properly. If you have all of your individual instances (.vep) saved separately, rebuilding your Server Project (.vesp) after it goes kaput is quick and easy. If you don’t have individual instances saved separately, you can find yourself SOL.
Aside from those general points, I’m afraid I don’t have a clue why your strings would play when auditioning notes, but not while pressing play. I’m curious though, if you don’t mind answering a couple questions, just because I’ve sometimes experienced weirdness that was somewhat similar:
– When you click a note and hear sound, do the audio level meters bounce in both the VI player and the VEP channel strip? Or as sometimes happens to me, do the level meters only light up in one or the other? Or do they not light up at all?
– Similarly, there’s a little dot on the VEP channel strip (in VEP6, might be in a slightly different place in VEP7) that lights up to show it’s receiving MIDI activity. Does this light up even when you get no sound from the strings, or only when you do get sound?
Now that I’m thinking about it, a couple other quirks come to mind (for all I know, these only apply to VEP6; I haven’t tried VEP7):
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Do you have any of your channel strips placed inside a folder? Sometimes this causes weirdness for me, and sometimes the solution is to drag them out of the folder, and if you get sound at that point, you may still get sound after putting them back inside the folder – or sometimes you won’t, but then reset the SMC and in my experience, this gets fixed.
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Are you running MIR Pro by any chance? I’ve experienced some very weird behavior sometimes, which as far as I can tell, has abated after I uninstalled MIR several weeks ago. I don’t really know the pattern here, but I have a hunch that when running MIR and having Kontakt in a VEP instance, they sometimes interfere with each other somehow.
Sorry this was somewhat scatterbrained but I hope your strings get back to normal soon.