I have a sweet little c/c pen that I tend to keep in my pen rotation which is a first-time-every-time writer no matter how many days or even weeks since I used it. It's not a vanishing point, but the nib & feed are on the smallish side. I have to keep the nib unit thoroughly flushed out on a more frequent basis than any of my other pens or it stops being that reliable first stroke writer.
Vanishing Points have such small nib/feed units, so perhaps a thorough flushing and or soaking of the nib units on your pens is in order. That is, if you haven't already done this.
So I would pull the nib unit out, remove the converter, and give the nib unit a good flush with one of those squeeze bulb syringes. Then I would soak the nib unit in a glass of water for a day or two and flush again to make sure there isn't dried ink restricting good ink flow. I would repeat soaking until flushing results in clear water running through the nib.
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