I have hiked the whole Vosges mountain range from the German border to Belfort. The Northern part of the Vosges is still below 1,000 m and Douglas firs dominate. The further South you go, the higher the mountains and the more daily climbing you will have to do. Also, a lot of places along the crest are not forested and therefore very exposed. This is lovely with breathtaking views if the weather is nice - but very uncomfortable when the weather turns bad. On my second last day I had an almost complete white out situation due to fog!
The trail marking is decent but not great. You would get by without a GPS and only a paper map or guide book, but you might get lost a couple of times. No big deal, but it can be time consuming to navigate. Do not trust the times given on the sign posts - although mostly correct I have encountered various signposts that were one hour or more off!
Water is usually not an issue. There are lots of springs in the mountains, sometimes even real fountains. Most villages had beautiful public fountains with drinking water, but don't count on that. When you desperately need them, they are usually not there...
There are very few shelters along the trail. There are some huts (abri) that are permenantly open and they are very comfortable and luxurious containing even a wood burning stove and benches and tables. But there are not too many of them and they are never around when you need them. There are also official refuges run by ski and hiking clubs but unfortunately those are usually locked and only open on weekends. During the week they are completely locked and not even an emergency shelter room is open. On weekends you will see plenty of cars parked around them. Apparently they are more used for family and friend reunions than by hikers.
Although you will have to hike on forest roads and sometimes even on pavement a surprisingly high percentage of the route is on single file trail. The Southern part of the trail follows a tourist road called "route des cretes". I normally hate road noises when I am hiking but in the Vosges this was not much of a problem. There is very little traffic on the road (mostly tourists) and very often you don't even see it. Only on weekends noisy (German) motorcyclists can be a bit of a problem. This seems to be a very popular route for bikers - but also for cyclist. So if you are after a pure nature experience the Northern part of the Vosges is better where the GR 53 and GR 5 are far away from roads.
And last but not least: Due to the high altitude there were hardly any ticks! Only a few in the North, but none in the South.