I am talking about the Vedic commentator Sayana.
Was he known to be a Vaishnava?
Sayana was an Advaitin and brother of Madhava (generally believed to be pre-ascetic name of Sri Vidyaranya). Sri Vidyaranya played a crucial role in formation of Vijayanagara Empire.
It was at this time that the two brothers, Madhava and Sayana, the sons of Mayana of Bharadwaja Gotra, approached Sri Vidyaranya and sought his blessings. Sri Vidyaranya graciously gave them his unfinished Veda bhashyas and some other works. He blessed them to complete the works in their own names as Madhaviyam and Sayaniyam. Later, both the brothers served as ministers in the Vijayanagara empire under Bukkaraya and Harihara I and II.
When Sayana comments on a philosophic hymn, he interprets it according to the view of Advaita Vendanta. Thus he explains
Rigveda 1.86.10 as follows :
'Destroy the positive ignorance which is in the cave (heart) of the body. Remove all the desire, anger etc. which devour the values ot life. (Purushartha). Give the light of knowledge in the form of the realisation of the Highest Reality
While commenting on the Creation Hymn (Rigvedax.129) the word asat as,
'indescribable like the horn of a hare' and he explains the word sat as 'which can be described as existent like the Atman'. The word svadha is explained as maya (Cosmic Power).