There are messenger bags, and there are messenger bags. What sets the ApeCase ACPro1610W apart from the others is the unique features it has. While this bag is considered a large bag, it is still small enough for your vacation needs. I often say that a bag large enough to carry everything is often too large to carry.
The bag can carry a DSLR, a couple of lenses, and a few accessories - just perfect for vacation and travel when you don't want to carry a bunch of stuff.
The bag has several unique features, not normally found on messenger bags of this type. First, there are protective side flaps under the top cover that help isolate the contents of the bag. This will help keep rain and dirt out of the bag, as well as preventing anything falling out. While it is probably not 100% waterproof, it should help in this regard. And since there is a rain cover as well, the side flaps will certainly help keep water out until the rain cover can be deployed.
The rain cover also has a unique feature in how the bag handles the D rings for the shoulder strap. There is a slit in the cover to provide access to the shoulder strap with "velcro" tabs that secure the ends. In contrast, many bags of this type have no provision whatsoever for connecting the shoulder strap with the rain cover on.
The bag also has an expandable section that allows increasing the depth of the bag by 2" for additional room. However, remember my caution that a bag large enough to hold everything may be too heavy to carry.
Also, the main camera storage module is removable so that you can store your camera gear separately and/or use the bag for other purposes.
In addition to the top zip compartment, the entire bag's contents can be accessed without taking the bag off your shoulder. This is the main advantage of the typical shoulder/messenger photo bag.