If one has to take medical pills during Pesach, although even if they contain Kitnios we usually allow them in need, yet I wonder if it would be preferable to take them with a drop of vinegar or any other distasteful liquid, to make them even more permitted?

Talmud (Pesachim 115a) mentions that one should not consume matza and maror when they are enveloped or mixed together, since the mitzva of matza is even in our days Biblical, while the maror is only Rabbinical. Minchas Chinuch (M.6 : 2, as explained by Minchas Yitzchok 7: 32) maintains that the above is only in regard to the complying of two different mitzvos, but it does not apply to prohibitions. Hence, we do not permit the consumption of a prohibited food just because the taste became bitter by adding different spices or condiments (unless it became totally and irrevocably inedible). Therefore, adding vinegar to the pills one takes would not make much difference in Halacha.