Definitely some people are with me doFirst thought was "ah interesting" as my wife gets bitten a lot. Then I saw the website and thought "uh-oh, sounds a bit alt-'med' to me", also not entirely unbiased.
Pubmed is a pretty good source for scientific medical research. The conclusion from a study there is that some people simply taste better than others to mossies, but consuming vitamin B doesn't make any difference. I would guess that's also the case for horse flies, but couldn't (quickly) find any scientific studies.
While taking excessive amounts of some forms of vitamin B can be harmful, there appears to be no current conclusive research to suggest B1 (thiamin) would do much to you other than cost money. See report from the NHS.
n't get bitten , or the bites don't bother them. More midges I'm on about though. I get swollen lips eyebrows etc for a few hours. I don't always know what has bitten me unless it is midges.