This alternate version of Ramayana has many unique legends which are not mentioned anywhere in Puranas like Padma Purana, Skanda and Bhagavata Purana. (I took these names because these puranas have narrations of different versions from Valmiki Ramayana).
From Ananda Ramayana Sara Khanda Sarga 8
yadyāpi tvam durācāro nihato asi raṇe mayā
Yes, it is true that Vali himself ordered Sugriva to stay vigilant at the entrance of the hole, through which Maayaavi entered.
इह तिष्ठ अद्य सुग्रीव बिल द्वारि समाहितः |
यावत् अत्र प्रविश्य अहम् निहन्मि समरे रिपुम् || ४-९-१३
Then Vali said to me, "Now you stay here on the alert, Sugreeva, at the entrance of this hole till I return on destroying ...