Is VPK a preschool graduation? Or elementary?
Because to be honest....the one graduation that matters is when a child is 18. Sorry, I know it means the world to your little one right now, but the finals mean the world to your older child too.
You will have loads of these mini-"graduations" to attend and they will conflict with other things in the future. Your older child has worked hard to get to the finals of a sport she values and your older child also will be much more aware than your younger one that she is being slighted if she is told, "You must miss the event for which you have cumulatively worked, over many months, to attend a preschool event."
I would undoubtedly let the older child play in her finals and definitely would have one of you parents attend the finals too. One of you goes to the graduation, the other to the final. You cannot all be at every event, and it is not required that both parents and every sibling attend every event; tag-team this and let one parent do each. But if you ask the older child to forego her finals -- well, it smacks of telling her, "Your commitment to your team, and your work over months of time, doesn't really count as much as this other event that will be happening again, by the way." Your younger child will be fine if sister does not come, but your older child will sit through the younger one's graduation hurt and fuming and feeling she has let down her entire team. And resenting her little sister, too.
You CAN do both if you just send one parent to each event. This is going to come up again and again so now's the time to set expectations with yourselves and your kids about this: The whole family does not have to do everything together unless it's a truly life-changing rite of passage. It's the only way you will keep your kids from resenting each others' activities.