Sony has released official specification sheet, which states that Vita has 512MB RAM. You can check it below.

Official Specs:

Platform Name PlayStation(R)Vita
Model number PCH-1000 series
CPU ARM(R) Cortex(TM)-A9 core (4 core)
GPU SGX543MP4+
External Approx. 182.0 x 18.6 x 83.5mm (width x height x depth)
Dimensions (tentative, excludes largest projection)
5 inches (16:9), 960 x 544, Approx. 16 million colors,
Screen OLED
—— ——————————————————
(Touch screen) Multi touch screen (capacitive type)
————– ————————————
Main memory 512MB
———– —–
VRAM 128MB
—- —–
Rear touch pad Multi touch pad (capacitive type)
Cameras Front camera, Rear camera
Frame rate : 120fps@320×240(QVGA), 60fps@640×480(VGA)
Resolution : Up to 640×480(VGA)
Sound Built-in stereo speakers
Built-in microphone
Six-axis motion sensing system (three-axis gyroscope,
three-axis accelerometer), Three-axis electronic
Sensors compass
——- ——————————————————
Location Built-in GPS (3G/Wi-Fi model only)
——– ———————————-
Wi-Fi location service support
——————————

Keys / Switches PS button
————— Power button
Directional buttons (Up/Down/Right/Left)
Action buttons (Triangle, Circle, Cross, Square)
Shoulder buttons (Right/Left)
Right stick, Left stick
START button, SELECT button
Volume buttons (+/-)
Wireless
communications Mobile network connectivity (3G/Wi-Fi model only)
IEEE 802.11b/g/n (n = 1×1)(Wi-Fi) (Infrastructure
mode/Ad-hoc mode)
Bluetooth(R) 2.1+EDR (A2DP/AVRCP/HSP)
———————————–
Slots/Ports PlayStation(R)Vita card slot
———– ————————–
Memory card slot
—————-
SIM card slot (3G/Wi-Fi model only)
———————————–
Multi-use port (for USB data communication, DC IN,
Audio (Stereo Out /Mono In), Serial data
communication),
—————————————————
Headset jack (Stereo mini jack) (for Audio (Stereo Out
/Mono In))
———-
Accessory port
————–
Power Built-In Lithium-ion Battery,
—– —————————–
AC adaptor
———-
Supported Music
——— —–
— MP3 MPEG-1/2 Audio Layer 3, MP4 (MPEG-4 AAC),
AV content WAVE (Linear PCM)
———- ————————————————-
format Videos
—— ——
— MPEG-4 Simple Profile (AAC), H.264/MPEG-4 AVC
High/Main/Baseline Profile (AAC)
——————————–
Photos
——
— JPEG (Exif 2.2.1), TIFF, BMP, GIF, PNG

Source