This mode may be extremely rare to some Claptraps and is one of required modes to complete achievement "That helped, right?". After googling a lot and experimenting even more i found out a nice recipe to force Rubber Ducky fairly quickly and probably guaranteed way. Here is how it goes:
1. Go to Concordia (to speed things up)
2. Reset skill points and use only one point just to enable VaultHunter.exe
3. Remove shield and grenade mod, optionally detonate few grenades right under your wheel to lower your health (rumor has it that low health improves chance to get Rubber Ducky)
4. Go to edge as i did on a video - this is clearly just to speed things up, for example if you throw minion trap or holo trap then they will fall into bottomless pit and you will be able to use vaulthunter.exe again instantly, without waiting full cooldown time to restore it.
5. Spam VaultHunter.exe until you get Rubber Ducky.
6. Have fun jumping all over the place :-)

On first attempt with this method it took me 6 tries to eventually get Rubber Ducky. Recorded one is second attempt which i captured just for you :-) and as you can see this time it went even faster and Rubber Ducky spawned on second launching VaultHunter.exe.

You can watch how using above recipe i was able to spawn Rubber Ducky.
Watch this video on youtube