I would advise not investing in Bitvest.io. They do not portray how much of a cut they take from investors anywhere on their site, in fact you need to locate their bitcointalk thread to find that.
House edge: 1.8%
Their cut: 0.65% of every bet wagered
True HE for investors: 1.15%
Now for margin. They allow margin investing, sounds like it would be a benefit for investors... Well it isn't. Because just about every investor utilizes it (meaning your % share of the bankroll does NOT increase, thus your profits do not grow bigger). What this does is open you up to dangerous variance. Such as the bet on April 26 on Bitvest that wiped out over 15 BTC of the real invested amount (or 15% of the total bankroll). Another site that operated this was was SafeDice... Which got hammered and lost over 90% of the user's bankroll from a single dice shot.
This site also has low volume. 2800 total wagered is nothing, especially for a site that has been open this long. Less volume = less profit for investors.
Bitvest only has like 30 investors most the profit was made around 30-40 btc invested.
Most of it has been made since February not October an the lack of profit is do to lack of user interest.
Bitvest was more worried about building a higher quality product then massive profits.
An many of the investors who joined at the start i'm pretty sure cleared 100% on their investment doubling it.
As for margin based investments, the limits go up an down there is no way to win that much, first big hit the limits drastically change.
I'm talking about oh you can win 14 btc they hit it well now you can only win 4 btc because the massive drop.
In the short term you might lose some but its no where close to 90% they will hit edge before that happens as the gi go's down so does the amount they can win at one time.
Also the house edge is 1.7 not 1.8 the house splits it as 1.15 to investors an 0.55% to site an only on gamblers loses.
The rest of the bet go's fully to investors so every bet 99.45% of all loses go to investors.