Tori and Trina Vega's Grandmother(s) has never been seen in Victorious. It is possible that both mentioned are different, as Tori has mentioned that her grandmother is deceased, and later it is mentioned that she is going to see Tori and Trina on Thanksgiving.

Mentions

Season One

Jade Dumps Beck

She is mentioned in Jade Dumps Beck, where Jade sobs into a pillow that the grandmother knitted. It is known that she is deceased. Jade gets a little bit of make-up smudged on Tori's pillow and Tori said "I'll just ask my grandmother to make me another one. If she ever comes back to life."

Tori the Zombie

She is also mentioned by Trina in Tori the Zombie when she says she is not afraid of anything ever since she saw her grandmother naked. However, the girls may have been talking about 2 separate grandmothers.

Tori & Jade's Play Date

Trina is seen talking to her grandmother on the phone about her date and then hangs up on her because she "doesn't have time to hear about (her) pain."

TheSlap.com

In the #ThingsYourGrandmaSays Trending Topic, Tori says that her grandmother would say for Tori to "Please tell Trina to stop asking if she's in the will."