Modoru Vita

modoru means "to go back" in Japanese and is a downgrader for the PS Vita™.

Requirements

Your device must already run HENkaku/h-encore on firmwares 3.60-3.74 in order to use this software.
Your device's battery has be at least at 50%.
All your plugins must be disabled, therefore you will not be able to launch the downgrader from a SD2VITA and hence, you must have a Memory Card (or Internal Storage).
If you have installed IMCUnlock by SKGleba, it is recommended to uninstall it first before attempting to downgrade to a firmware lower than 2.10.

Download and install modoru.vpk using VitaShell.
Obtain the PSP2UPDAT.PUP file of your desired firmware (make sure that this firmware is officially hackable) and place it at ux0:app/MODORU000/PSP2UPDAT.PUP (don't install modoru.vpk afterwards, otherwise the update file will be removed).
Disable all your plugins. Easiest way is renaming ux0:tai and ur0:tai to some other name.
Reboot your device and relaunch HENkaku/h-encore.
Launch the modoru application and follow the instructions on screen.
Enjoy the installation and welcome to your favourite firmware.

A: Here is a nice collection by darthsternie: PS Vita Firmwares. Make sure you download the firmware from the Complete Official Firmwares section.

A: There are packages with (pre), (systemdata) and (full). You should choose the full one.

Q: My factory firmware is higher than 3.65 and modoru doesn't allow me to downgrade to 3.60/3.65.

A: Unfortunately, there are some devices with factory firmware above 3.65. These cannot be downgraded (yet). This means no bootloader hack for you.

A: Yes, that's the main goal of this tool.

A: It has not been tested yet, but you can very likely do it. You should even be able to go lower than firmware 1.692, which is officially inhibited.

A: You can go down to your factory firmware (this is highlighted in yellow within modoru).

A: Yes, you can downgrade, update or reinstall any firmware using this tool.

A: Not likely, since this application is using the official updater and only makes a few harmless patches to bypass some checks.

v2.3

v2.1

v.1.0

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Thank you!

Thanks to Freakler for the LiveArea design.
Thanks to liblor for the name suggestion.
Thanks to yifanlu for prior research on downgrading.
Thanks to molecule for SCE decryption utilities and secure coprocessor write primitive.
Thanks to SKGleba for betatesting.