VESA mount adapter upgrade kits are not available for iMacs with a stand. When an iMac is ordered with the built-in VESA mount adapter, the stand on the iMac is replaced with a VESA mount adapter and the iMac cannot be used with the standard iMac stand.
the VESA model iMacs do not include a stand, and is a separately configured model. http://store.apple.com/us/buy-mac/imac-vesa?product=Z0QF&step=config
If you do want the VESA model, then return the iMac that you have, and replace with the VESA-equipped iMac.
Apple will easily do that within the first 14 days after purchase.
Unfortunately you cannot retrofit your machine. If it is less than 2 weeks old and you bought it directly from Apple then you may return it and then order the CTO model with the VESA mount.
Starting with the late 2012 (laminated screen/slimline) iMacs, the vesa mount is a factory installed itemonly, that replaces the regular stand, and is a $40 option. You then need to purchase a mounting arm to attach it to.
Sorry, but there's no ready made solution to convert after the fact.
Not sure what you mean by "portable compact, with the leg it is too large to fit bag etc"? It is a desktop we're talking about, that you are planning to permanently mount to a wall-mounted, or desk-mounted arm.
The above suggestion by coldsweat, while not a very elegant solution with the stand still attached and dangling in mid-air, will work however, if you're determined to mount your 2013 iMac to an arm.