I noted your update about going out of season and this is a really good thing - check it isnt a French holiday while you are there though as we found still a LOT of day guests in the pool at the weekend. Still loads of room to swim but you'll have virtually no queue for the slides on the quieter weekdays.
I read a lot of the recent bad and good reviews on Trip Advisor. A few overall comments: European centre parks are not identical to the UK - much cheaper and the activities timetable can be limited. Adjust your expectations and all is fine. I didn't see any rats or evidence of rats. Our apartment had been refurbished and it was in lovely condition.
The wider park area is allowed to be wild, it isn't manicured (extreme contract with neighbouring Disney where I saw plastic flowers attached to real plants!). The animals on the farm appeared to be well cared for which was heartening. It's on a huge lack within a woodland area so... insects, birds and I suppose rodents although i didnt see any.
I did see a piece of glass by the bins, which was not cleared up quickly. Guess what next time I went I put it in the bin.
With experience of European centre parks under our belts, we planned to drive to VN, arrive in the afternoon and swim before our apartment became available in the afternoon. So we had our swim kits in a separate bag ready in the car. Checkin was very easy. At 4pm the lock mechanism on the door shows a green light and the wrist-watch doorkey fobs will allow you access to the accommodation. Pretty much everyone park right outside their accommodation to unload.
We didn't have any need to contact customer services, use the laundry (I washed by hand in the apartment), or use the left luggage area, or use the bus. So I think we were lucky bass on the reviews i read. Note the bus I believe is a regular bus company it isnt a Disney or VN bus, so the rudeness of the driver not really VN's fault. We only did a few paid activities (best was a brilliant woodland high ropes area with a choice of courses you could choose your level of difficulty) and had no problem organising it, the staff supervising were very helpful, careful and kind.
I read a comment there is "nothing to do with under 3s" which is weird. My 3 yo loved the farm (free), indoor play (free), and playground in the woods with a big sunny sandpit adjacent (free). Not to mention the huge swimming pool (free). He also loved simply scooting all over the park. I'm not sure what else you want for a little one. We didnt expect to dump him in a kids club so didnt even research if it was a possibility.
We also planned to shop at Auchan not Franprix. Only an idiot complains about prices in a Centre parks onsite shop, they are notorious! We were disappointed by Vapiano with it's crazy long queue but found restaurants outside VN were good for an occasional big meal out and just self-catered our other meals. I know it seems mad to go offsite for restaurants, but we had our car and it was just a short drive.
Overall I would not hesitate to return, but I wouldn't go in the summer holidays because of heat and the crush of people I expect you'd find... with the park fairly quiet the maintenance staff were keeping on top of things nicely it seems.
Last thing to say: we had two toilet rolls, one for each toilet in our apartment. Depending on how much time you are in the apartment you may need to take a spare.