The answer is both yes and no. It depends…
No, you can’t manipulate a totally different VST plugin with HALion. You can’t easily manipulate ‘MULTIPLE separate instances’ of HALion from a single ‘master’ instance either (might be possible with some LUA scripting to have different ‘instances’ communicate, but not something a beginner would be doing out of the box).
What you CAN do is more deeply manipulate/edit HALion’s own programs/sounds from within a given HALion instance.
For some of the content that is included with HALion, yes, you can deep edit variables regarding tremolo and vibrato. HALion does ship with some really nice content from the HALion 4 era that is fully ‘unlocked’. Those sounds don’t have a fancy Macro Editor either, but they are high quality, and among the best to ‘study’ to see how go about building your own instruments.
There are also a number of sounds with locked layers (Basic, Artist, Hybrid, Sonic 3 Factory Content, and more), so while you can reuse such layers in new sounds of your own and work with whatever parameters such layers have ‘exposed’ (usually through some quick controls at the top of the layer), you can’t really DEEP edit everything about them like you could with your OWN content starting from scratch.
Some sounds that ship with HALion might allow you to deep edit them, BUT, the vibrato/tremolo effects are recorded into the sample itself. In these cases, no, you cannot ‘remove’ the vibrato or tremolo effects that are baked into the sample itself.
Most if not all of the solo string samples that ship with HALion have vibrato as part of the sample itself. As in, when the samples were made, the player implemented a real vibrato technique on the instrument. If there are pure solo string sample with ‘zero’ vibrato included with the HALion factory content, I haven’t found them yet. I suspect the closest thing to it might be found in the newer ‘Studio Strings’ library.
If you can find a pure ‘non-vibrato’ sample in all the provided HALion content (or make samples of your own…perhaps by resampling some other library/keyboard/etc), then YES, you can indeed design programs in HALion where the user can control LFO/Envelopes/ETC for pitch and dynamics in real time (via VST parameters and/or MIDI CC events). As an example…I needed a ‘pure, no vibrato’ solo violin at one time. I grabbed some samples for this from an old Roland Sampler disk (and also resampled some from a Fantom XR), and was able to build exactly what I wanted. I can bring a kind of synthetic vibrato (LFO or a Looped Envelope applied to ‘pitch’ with some controls of that linked to respond to the CC I want) in and out with the mod wheel. Also, I made it so different key velocities open/close various filters. Have also experimented with LUA scripts snooping the tempo posted by the host (if the host provides it) to manipulate some concepts like ‘body/air’ (bow pressure) differently according to the current ‘tempo’.
If clean strings alone is why you’re considering an upgrade, I don’t think you’ll find it in HALion. You might be better served by getting a dedicated string Plugin/Library.
If you have access to a lot of raw string samples (or can make your own) and want to BUILD sounds from the ground level up, then upgrading to full HALion might be of more interest. Yes, if you have clean samples without baked in vibrato/tremolo, then you could build very powerful sounds where all that stuff could be modulated by the user (by live switching samples, using LFO/Envelops to wiggle pitch/dynamics, etc…all of which you could bind to controls of your choice…pedals, aftertouch, mod-wheels, breath controllers, and more).