This paper proposes an internal reconstruction of the vowel system of Pre-Khroskyabs by analysing bound state apophony in modern Siyuewu Khroskyabs. While most modern Khroskyabs dialects do not exhibit sufficient variations of bound state apophony, Siyuewu Khroskyabs is conservative in this regard. The internal reconstruction deals with the different reflexes between final and non-final vowels in modern Khroskyabs dialects, and postulates a unified origin for them, by hypothesising two series of vowels, velarised and plain. Velarisation in Pre-Khroskyabs is then compared with uvularisation in Tangut, showing regular correspondences between the two. This work demonstrates that velarisation is an important feature that can be reconstructed back to Proto-Gyalrongic, and that it helps us in understanding Trans-Himalayan historical linguistics. Additionally, it can be seen through this paper that internal reconstruction is an indispensable procedure in reconstructing the history of a language, as a significant portion of vocalic contrasts cannot be reconstructed by cross-linguistic comparisons alone.