Ok, in that case you'll have to delete the data partition. However, if you take it out I have no idea if you can get it back in Vista again. This is what I would try but I can't guarantee it. You may also want to backup any data before you start playing around with partitions just in case.
If you have any data in your data partition (which it looks like you don't) copy/move it over to your C drive.
In Vista delete the data partition. You should now be left with a big lump of unallocated space in the middle of your drive.
Boot up the LinuxMint live CD. In the liveCD mode go to menu>administration>partition editor to load Gparted
This is where you want to be careful what you do. Make sure you don't accidently overwrite Vista.
In Gparted you will see a list of all the different partitions on your computer. Don't worry that your C partition is called something else in Linux, it'll still be the 115GB ntfs partition ringed in blue just with a different designator.
Click the grey "unallocated" line so that it is highlighted. You should now see the "new" button appear if you've clicked the right one. Delete/resize should still be greyed out.
Click new, and a popup box appears. Change the "create as" box from primary to extended. You can click and drag the edge of the cyan box so that it fills the whole unallocated space. Hit add. You might need to click apply in the top toolbar before you can do the next step.
You now have an extended partition you can add logical partitions into in the same way as you've added your extended partiton. You need to add at least 3 partitions in the following order:
1. Swap partition: If you're using 32bit Ram+swap maxes out at 4 gig. You might want to look around the forum for swap size advice to work out what's best for you. Standard "old" advice is 2X ram but if you've got Vista you've probably got enough not to really need the swap space, however, you have to have a swap partition because linux expects it. For 2GB ram 1-2GB swap will do. For 4GB ram call it 1Gb just to be safe as you've got a lot of hard drive space to play with. If you're going for 64 bit or might in future maybe add a bit more swap.
Enter your chosen size in MiB and change filesytem to linux-swap.
2. LinuxMint partition: 12GB will give you loads of space as all your data is in another partition. Filesystem should be ext3. You may want to create a couple of these so you can try other Linux distributions in the future.
3. Data partition: Make this the size you think you'll need for your data. Any size except 115GB will do so that you don't confuse it with the Vista partition. Filesytem should be set to ntfs so Vista can read it. (Hopefully).
Once you've finished creating partitions quit and boot into Vista to see if you can use the data partition. I don't use Vista so I've no idea if it will pick it up automatically, if you have to use the editor or something else. You might have to google it or post on a Vista forum for a solution.
Once you've determined whether or not you can use the data partition in Vista or not it's time to install Mint. Reboot into the liveCD and install up to the partition editor step. Choose manual partitioning. You should now see the partition scheme you created earlier.
First select the small swap partiton. In the bottom right hand corner there is an edit button. Click it and choose the filetype to be swap and tick the "format" box (It might automatically tick a partition designated as swap).
Secondly select the 12GB partition to install Mint into. Again hit edit. This time select filetype as ext3, tick the format option and select the mountpoint as "/".
Lastly select the ntfs data partition. If you can use it in Vista select ntfs. If it turns out you can't use it in Vista you might want to change to ext3. Do not format this partition if it's staying as ntfs. You need to type in a mountpoint for this partition. I suggest you mount it as "/home/<yourusername>/data" replacing <yourusername> with whatever you're going to use as your username. Note that your username will be in lowercase. For example I have 2 data partitions mounted at /home/dave/documents and /home/dave/multimedia.
Double check the partitions and options you've selected and continue with your installation. Remember to use the same username later when you enter your details.
Hopefully everything should run smoothly, good luck.