I saw your post on Facebook on my incredible 26887 bike ride and you're right, that is an Edge 520.
The Vivoactive is very much the jack of all trades, master of none - it's a mix between a smart watch, a fitness watch and a fitness band and I'd say it's not great at any of those but it's a good enough mix. As a smartwatch its functionality is very basic and its screen is quite low resolution, there are a few very basic applications but the only 'smart' function I use it for is really the notifications - it essentially just pipes the data in the notification bar on your phone to the screen which works most of the time but there's annoying quirks such as receiving two messages from the same person means it shows 'two new messages' instead of the message and Smartglass messages just show 'someone sent you a message'
As a fitness device it's limited in that it only does 'smart recording' where it spaces out the GPS points depending on your speed whereas the Edge offers per second recording so it means you get more jagged lines like this:
Rather than following the trail like the Edge 800 did:
It also lacks a barometric altimeter and instead uses GPS for altitude although to be fair the difference in the overall stats wasn't that much compared to the Edge 800. The bigger issue is that I like to be able to see the data as I'm riding and the VA screen is small and difficult to read during an activity so I tend to only use it when I don't have an Edge.
On the plus side the Vivoactive is quite small and light compared to other watches, the display is on all the time and uses ambient light - this means its power efficient and easily readable in the sun although it can be dim indoors, it does have a backlight which can be automatically triggered on an notification which works fine. The batterylife is superb in normal use (GPS hammers it quite a bit) and it usually manages 7-10 days with ease, I hadn't dug out the charger after being on holiday and it managed around two and a half weeks I think before it went flat.
I'd double check the Garmin forums to see if anyone is using a VA with your phone as it seems quite variable in its performance with phones, on my Note 4 it works perfectly and keeps a solid connection however at one point there was a dodgy FW on the phone and it kept dropping the BT connection which was maddening - I was close to getting a different smartwatch as I missed having it and the regular dropping made it useless. If there's any issues with your model of phone and the VA, I wouldn't touch it with a bargepole.
With regards to the Edge 520 vs a mobile phone, I guess it depends what you want from it. I originally bought an Edge 800 with the intention that I could record and follow trails as I go out in a group but then could never find where they'd gone when I went myself afterwards. In that role the 800 was largely rubbish as the screen is just too small and too low resolution, I did look at an Edge 1000 but I disliked the capacitive screen (since they're not great with gloves and rain). I went for an Edge 520 as I wanted the bluetooth functionality (It shows calls, texts and you can let someone track you online although it's basic compared to the Vivoactive) and to be able to use the Varia rear radar system.
For the road bike I find the Edge very useful as it means I can keep an eye on my speed, distance and particularly the gradient to ensure I keep a good cadence - without the Edge I find I don't take advantage on downhills by taking it too easy and on hills I push too hard so by keeping an eye on my speed and gradient I can adjust my gears as necessary which for long rides makes a noticeable difference. The rear view warning radar system is handy as it picks up cars at a good range, shows me how fast they're approaching and how many there are so if they're coming up fast behind me I know to get tight into the side of the road as I know they won't give me room. I realise you shouldn't have to do that but it's the reality.
On the mountain bike I don't find it quite as useful, the main use is to track the route and for livetracking which I don't need the Edge for, I tend to just glance at it for the time, temperature, how far I'm out and how much I've climbed. On races though it's very useful as I need to keep a good pace so being able to keep an eye on the time, distance, speed, elevation etc. means I can pace myself much better round each lap.