Opening files of size larger than RAM, no swap used.

I found myself asking this question while doing small experiments and reading upon how memory is managed in Linux.

I have a 64-bit Centos 6.5 machine with 512M RAM and 1G SWAP. I created a 1GB file '/mytest' (using dd) and opened the file with vi. Once the file was completely opened, I opened another terminal to look at the memory usage. As expected, the vi showed up at the top when the top output is ordered by memory usage. So I checked free -m expecting to see the swap usage going up, but to my surprise, the SWAP usage is NIL. Since the size of the file opened is twice the size of RAM, where is the file being held in memory?

[root@server ~]# ps aux | grep mytest
root     18940  8.5 53.3 287880 267156 pts/0   S+   09:25   0:28 vi /mytest
root     18976  0.0  0.0    364   144 pts/1    R+   09:31   0:00 grep mytest

[root@server ~]# ll -h /mytest
-rw-r--r-- 1 root root 1000M Oct 24 09:25 /mytest

[root@server ~]# free -m
             total       used       free     shared    buffers     cached
Mem:           489        481          7          0          2        140
-/+ buffers/cache:        337        151
Swap:         1023          0       1023

Swappiness is turned ON and is at the default value 60. And in the top output, memory usage under VIRT and RES is not adding upto around the 1G as I was expecting.

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
18940 root      20   0  281m 260m 1172 S  0.0 53.3   0:28.85 vi /mytest

I even scrolled through the opened 1G file to see if the swap usage is going up only when I am scrolling (used sleep in the other terminal), but still the swap usage was nil. It now looks like my whole understanding of how memory and swap works is incorrect. Can someone throw light to how this huge file is held up in the memory when the size of the file is twice the memory size?