dmitri,
benzothiophene does not react like indole. Sulfur does not act like nitrogen. Their chemistry is completely different! Does nitrogen act like oxygen? Some research brings up a number of papers on this subject. Regarding the vilsmeyer, I refer you to W.J. King and F.F. Nord, J. Org. Chem, 1948, 13, 635. The 3-chloromethyl route has better yields, but has 3 steps and 3-chloromethylbenzothiophene is a severe allergin. Do you think that titanium tetrachloride and carbon disulfide were used for fun? Harsh conditions are necessary for this conversion. The Rieche formylation is definitly the way to go. Rieche, A., Chem. Ber., 1960, 93, 88
Z. Chem, 1964, 4, 401
If you are interested in this reaction the most recent papers are: Buu-Hoi, J. Chem. Soc. (C), 1969, 339.
Clark, P.D., Irvine, N. M., Phosphorus, Sulfur and Silicon, 1996, 118, 61-77.