The thing about the scene was that Gulaal was thinking about Vasant but at the same time she did not have that far away dreamy look. A few episodes back when she was in the jeep with Kesar and her friends...that was a crystal clear fb about Vasant because she was staring into empty space and thinking back to the song she had sung, automatically connecting it to Vasant.
I was watching this dhol scene and yes she was thinking about Vasant BUT she was looking directly at Kesar...I don't know if that makes sense at all! So, her fb had not taken her away from the present...which is watching Kesar play. In fact, her shock was not that she was thinking about Vasant but the fact that she is staring at Kesar with this look of joy, all teary eyed and connecting with him through the music! Not once during the entire scene did I see her drift off, her eyes never left Kesar..
Her shock was obviously due to some hidden feelings because when you think about it, why would she be embarrassed if someone caught her remembering Vasant? Even in the jeep, while singing her song she had been remembering Vasant and when she was done she openly wiped her tears...no shock or embarrassment.
At the end of the day Kesar is Vasant's younger brother and both of them shared this passion for music. In fact Gulaal's friends had made fun of Dushyant because unlike Vasant, he was not a musical sort of a person. So, I would imagine that the music in itself would touch Gulaal on a much deeper level!
I was expecting her to dance as well but I suppose all of Manasi's scenes have been shot separately which might have presented some technical difficulties. Would have been amazing to see Gulaal and Kesar dance though!