Guys, lets be realistic here. Yes, it is 2 tickets and the OP would not be protected etc etc, BUT it appears to be the first VCE-FRA LH flight of the day, i.e. the aircraft would arrive from FRA the evening before and making any significantly delayed dept. from VCE very unlikely and the OP has 1h:40 min at FRA to get to the Z gate (to get from A to Z/B is quite easy). I would do it without hesitation (in particular if travelling with no checked bags). The chance that the OP will miss the US flight at FRA is extremely unlikely if traveling HBO.