Shani Deva's vahana is Grudha to be precise i.e a vulture and not a crow.
Here is Sanaischara Kavacham :
Asya Sri Sanaischara kavacham Maha manthrasya Kasyap Rishi , anushtup
Chanda , Sanaischaro devathaa¬
For the great armour of Sani , the sage is Kashyapa , the meter is
anushtup and the god addressed is Sanaischara.¬
Sam Bheejam, Nam Shakthi , mam keelakam, Sanaischara krutha peeda
parihararthe jape viniyoga.¬
Sam is the root , Nam is the power , Mam is the nail and this being
chanted to get rid of the problems created by Lord Sanaischara.¬
Now, refer to Dhyana Sloka (the meditative verse) given further below on the linked page:
Krishnambaradaram devam dvibujam gruda samsthitham,¬ Sarva peedaa
haram dhyayeth graham ganothamam.¬
I meditate of the best among planets who wears black silk,¬ Who has
two hands , who sits on a vulture and who removes all sufferings
Another Dhyana Sloka for Shani Deva is this:
Saurashtram Kashyapam Sudram SuryAsyam Chaturangulam|
Krishnambaram Grudhagatam Sourim Chaturbhujam||
Sulam Dhanur hastam SamAhvyeth |
Grudhagatam means one who moves on a Grudha or vulture.
The source of this above sloka is a bengali prayoga book that i have called "Brihat Vishuddha Nitya Karma Paddhati".
The book is of the same genre as Gita Press's prayoga book " Nitya Karma Puja Prakash".
In the Matsya Purana ,the description of Shani Deva is given as follows,which also says that vulture is his mount or vahana:
Indra neela dutih Shuli varado Grudha vaahanah Baana
baanaasanadharah kartavy -orka suthastathaa/
Shanaischara's body colour is of Indraneelamani/ blue diamond; he
rides a Grudha/ Vulture and carries bow and arrows, Trishula and