I think you may have two different problems going on here. One is that you're not getting the throughput you should expect given the Wi-Fi PHY rates you're getting, and the second is that you could do a lot better, Wi-Fi hardware-wise, than your current AP+STA (wireless router plus wireless PC) combination.
Insufficient throughput given PHY rates
If you're really getting the 12–24Mbit/s PHY rates on your Wi-Fi link, you should expect TCP throughput of about half that, so 6-12Mbit/s (so around 0.7–1.4 MebiBytes/sec). But this is only if you're using software that makes efficient use of TCP.
As a double-check, run a quick IPerf test. Make your VivoPC the IPerf server (-s), and your Win7 desktop on Ethernet the IPerf client (-c). Run a 10 second or 60 second test with default settings, then add -w 2M to both the IPerf client and server, to force a bigger TCP window on both ends, and see if it's any faster when the TCP window is forced to be plenty big.
If you get 6-12Mbit/sec running IPerf, it's an indication that whatever software you've been using to do your file copies has been inefficient.
Better Wi-Fi performance through better hardware
Your VivoPC claims it has 802.11ac, but it turns out it's using a RealTek RTL8821AE combo module which only handles a really crappy level of 802.11ac support known as "1x1:1" or "1 spatial stream". That means that even with a state-of-the-art high-end 2014 802.11ac "AC1900" AP, it could only do 433Mbit/sec tops, whereas most decent 802.11ac clients support 2-3 spatial streams, meaning they can do 2-3x that speed. I can't find info on what channel widths that chipset supports, or what the particular module in your VivoPC supports, so it's possible that it only supports 40- or 20-MHz-wide channels, meaning it may be as slow as sub-100Mbit/sec signaling, even though its technically 802.11ac.
You're certainly not doing it any favors by pairing it with a 2.4GHz-only, "N300" AP. Because your 1 spatial stream VivoPC can only do 1 spatial stream in 802.11n as well, so that means it's only capable of "N150". But on top of that, since it's a Bluetooth combo module, it probably insists on only using 20MHz-wide channel operation when in 2.4GHz, so that Bluetooth has some room to work (trying to use 40MHz-wide Wi-Fi channels in the 2.4GHz band hogs too much of the band and kills Bluetooth). So this combination of factors probably means that your VivoPC would max out at 65 or 72Mbit/sec PHY rates, which is barely better than 2002's 802.11a or 2003's 802.11g rates of 54Mbit/sec.
To get a lot better Wi-Fi throughput, consider upgrading your AP to an Asus RT-AC68U, which gives you state of the art 802.11ac with 3 spatial streams, 80MHz-wide channels, and high-powered transmitters, and still supports your legacy 2.4GHz devices simultaneously. Also disable the VivoPC's lame "AC in name only" built-in Wi-Fi, and instead plug in a USB 3.0 super speed, 2 (or 3) spatial stream 802.11ac adaptor, such as an Asus USB-AC56 (not the USB-AC53, which is hobbled by being USB 2.0).
