Woodstock,  I like your idea.  Depending on your Dad's awareness of the
holiday-have a separate celebration at his facility and invite Vince.
If your Dad comprehends the holiday on the actual day, then if your
sister wants just him over -that's her decision.  My experience with my
MIL was that she had no comprehension of the holiday(she did not know
what do do with wrapped gifts, could not recognize the contents and was
upset about all the "trash" lying around) and was anxious by the change
in routine.  For me, her confusion and agitation ruined the celebration
and exhausted me.  After that first year she was in the ALF, we now
visit her at her place instead.  3 years later, she's pretty much
oblivious to our visits.  Sad, but it does become easier to accept the
reality.
Beth

Woodstock, since your sister is too softhearted to consider other plans, it
may not be productive to suggest any to her, but since you ask, I do like
the idea of having separate parties.

You may recall that my father is quite elderly too (93) and bordering on
vascular dementia as well.   We occasionally do go to his house for events
and holidays, and then go out to the Chinese Buffet he likes so much which
is only a few minutes from his house.    But the actual holiday is spent
elsewhere, and we don't take him out to those events.   There is a good
reason for this.

He actually doesn't like going to events away from home anymore, and finds
all the "commotion" as he calls it, unnerving.   You might suggest to your
sister that going one day to his place, and the next having a family
gathering at hers, might work out well without offending anyone.   Being
around new infants and the crowd and attendant excitement might be harder on
him than he will admit to.

We have someone much like your father's "Vince" in the picture.  A lady who
was half of a couple my parents socialized with for many years who is also
widowed, was living in his home and helping him.  She took him to doctors
appointments and to the store and whatever.   This involved rather a lot and
we were all very grateful for her involvement.

Unfortunately along with her help came a lot of family troublemaking.   The
woman has turned out to be a very nasty and contentious person bent on
making difficulties with me and my siblings.   She recently moved out in a
huff and blamed me for something she thought I might have said, even though
she never actually heard me say anything.   In truth I have never said a
word against her, and have always been extremely kind and respectful to her.
This has left my elderly father in a bind, and also created a lot of bad
blood with threats to change the will, etc.

A lot of her resentment has to do with his miserliness and quality of life
issues that my father denied her when she lived in that house.   But none of
his children have control over any of that, though I am personally and
unfairly getting the brunt of his rage that she moved out.

So at the present moment I am dealing with a situation with some similarity
to what you are going through.   I can tell you that there is nothing in the
world that would convince me to bring my father in to family celebrations
that would include this woman, since she is an active troublemaker, not just
a boorish lower class person, which you seem to indicate Vince might be.

Fortunately I am not under any pressure from my father to be included in
holiday doings either alone or with the woman in question.   As I mentioned,
he prefers not to go out in the traffic on holidays and prefers to stay at
home.

I don't know if I ever want to even speak to the woman again, considering
all the trouble she started without cause at all, much less to welcome her
into our family celebrations, although I am notoriously easy going if
pressed about such things.   I'd probably cave in if I were in a pinch about
it, or especially if my father insisted.   I'd want to please him.

So speaking from my own side, I personally don't think your family should be
subjected to Vince's company if they don't enjoy him.  Is there any way you
could take your father to your sisters without Vince?   Can you use the
excuse that there won't be room?    Your gratitude to Vince for squiring
your Dad around should not necessarily include bringing him into the family
circle for holidays, especially since your sister will have two new
grandbabies and a very full house at that time.

Can you have a brief party one day at your fathers place with him which
includes Vince, and then do the full treatment with the younger generation
the next day at your sisters place without him?   Would your father go along
with that?   Would your sister?

I think you and your sister ought to think about other options now, see how
it could be managed without Vince, and if it turns out that you really must
include him, then just hope he gets lost in the family crowd and all the
holiday busy-ness and chalk it up to being nice to a stranger who helps your
dad out a lot, on a holiday, as so many people do that in keeping with the
season.