How to correctly configure AD9951 for 10Vp-p, 100Mhz sinusoidal waveform
First issue I see with your proposal is, the AD9951 is capable of 400 MSps. That means 200 MHz is just at the Nyquist frequency for this chip. You'll notice that no performance metrics are given in the datasheet for output frequencies above 160 MHz. If you want good performance (low harmonic content, etc.) you're probably better off finding a different chip with a higher sample rate, using some kind of mixing scheme to multiply up a lower frequency output from a DDS system to 200 MHz, or using an analog oscillator at 200 MHz. 

Second issue, using a transformer to bring up the output amplitude from ~ 1.8 V (peak-peak) to 10 V means the DSP output has to provide all of the power to the load. If your load is 50 ohms, that means your DSP will will be looking into something that looks like ~10 ohms, which it's probably not designed to do. You'll want to use some kind of amplifier here instead of a transformer.


  Since AD9951 powers up at 1.8V, I assume, the DAC (pins 20,21) would also output close to 1.8V?


See Figure 2 in the datasheet. The output is a differential current sink, and you "must terminate outputs to AVDD". On page 4 the full scale current range is given as between 5 and 15 mA. The voltage you get from that will depend on what resistance you pull up the outputs with. 

This also means with your idea of using a transformer to step up the output voltage amplitude, you'll only be able to deliver ~1-3 mA into the 10 V load.