In theory, mature VS's have upright growth habit and shouldn't "fan out" too much. If yours are mature enough the stems should grow out a few feet and then almost straight up, and should not flop with blooms, as opposed to younger ones. You're lucky to have found these in the first place - no one can seem to, let alone in 5-gal sizes!

Possibly transplant shock. Whenever the plants get moved from one location to another or from the plastic pots into the ground, they may react in several ways. Looks like yours is aborting the shrubs early. In its first Summer, it is very normal to see those reactions, and not just with SS. Too much heat makes them go from white directly to brown here at the plant nurseries in the Summer. Insert a finger to a depth of 4" near the root ball to sense when the soil is drying; give the plant 1 to 1.5 gallons of water per watering (more in the Summer) and do not be afraid to provide it with more shade than it currently gets if necessary. Maintain 2-4" of mulch up to the drip line.

You should use enough water to get the soil moist down to a depth of around 8". If you water the plant, wait a while for the water to drain and then feel that the soil is still dry at a depth of 8", you did not use enough water. On the other hand, if you test the soil and it feels wet or soggy then you used too much water. Another test: put some soil in between two fingers and press: if you see water droplets forming, you may be over-watering. If the blooms are prematurely going brown, the plant is heat stressed: temperatures are too high, the soil lacks enough moisture, it is too windy, it gets too much sunlight or the plant is too young and has a limited/small root system. If young, it may perform better in future years. In my extreme summers with nearly 100F daily temperatures, the blooms turn brown early. VS was developed in Gorron, a town in Mayenne, France with mild winters and mild summers. That is in the NW corner of the country; close to the English Channel and Great Britain.

Strawberry Vanilla's paniculata blooms are supposed to start white, turn different shades of pink (including the magenta typically seen in most pictures) and then turn brown. But it was developed in Gorron, France and our/your local weather can throw a monkey wrench in this color progression. You do not say where do you live or how much sun the original shrub was getting but if your growing season is short, for example, early cold weather can force it to shutdown the blooms (brown out) early thereby skipping some of the pinks. Down here, it tries to bloom at the worst time of the year, when our temps are above 90s and near the 100s... afterwards, it should turn pink but by the time it should turn pink, we are in daily 100s so the blooms turn brown and we get few or no pinks. To improve things, keep the soil mulched and as evenly moist as you can so it has no watering issues that could also trigger early browning. Give it as much sun as practical. Of the various hydrangeas, paniculatas can take the most sun, although down here, the leaves can suffer during the summer months so some afternoon shade in intense hot summer locations may be ok. I usually suggest to keep hydrangeas for at least three years as they can take about that long to become established in your garden. But, gosh, no pinks in three out of four years is not a good "record" for a paniculata. I am trying an early flowering one to see if pinks are not much of an issue. Paniculatas always produce blooms whose colors are not that sensitive to soil acidity, unlike your mopheads do... so there is nothing needed to amend the soil with when it comes down to the care of paniculatas. Just fertilize in Spring with about 1 cup (varies depending on the size of the plant) of organic compost, composted manure or cottonseed meal. Or use a slow release, general purpose chemical fertilizer with a NPK Ratio near 10-10-10 (follow label instructions... maybe even use half of the suggested label amounts if your soil is not sandy). Or even let the shrub "feed" off the decomposing mulch.