Hi Grodhisatva,
Great opening!
My suggestion is to only stretch the transistion time of the portamentos (I'd say the first 20 ms), so that the target note (played with vibrato) is not affected.
Make sure to switch the Stretch Mode ON, and use Algorithm 1 for the strings. If you change your settings, click "Render" to render a new patch.
Let me know which step helped.
Best,
Paul
Paul Kopf Head of Product Marketing, Social Media and Support