There is no availability round trip for those dates. On anything.

One way YVR-IAH on AC is 3,261 plus $121.23 but nothing direct and it leaves at 0615 hours. Later flights for 3,770. But again that's just one way.

And ugh... 4,312 for your return plus $52.14 on AC. Nothing direct.

No other carriers come up on the one ways for those dates.

If you depart one day early and return one day later you could get 6,706 plus $189.14 round trip. But again there is a stopover each way.

Here is the details for one of those options:

Thursday, December 21, 2017
Air Canada
AC 114
Air Canada
12:15 pm
Vancouver Intl (YVR)
7:35 pm
Toronto Intl (Pearson) (YYZ)
4h 20m
Cabin:Economy
Air Canada
AC 7715
Air Canada
*Operated by AIR CANADA EXPRESS - SKY REGIONAL
8:55 pm
Toronto Intl (Pearson) (YYZ)
11:34 pm
Houston (George Bush Intercontinental) (IAH)
3h 39m
Cabin:Economy
Airline Fare Rules
Round Trip
Per Person
1 Traveller
6706
reward miles
+ $189.14
taxes & fees
Tuesday, January 02, 2018
Air Canada
AC 8105
Air Canada
*Operated by AIR CANADA EXPRESS - JAZZ
1:35 pm
Houston (George Bush Intercontinental) (IAH)
4:49 pm
Calgary Intl (YYC)
4h 14m
Cabin:Economy
Air Canada
AC 229
Air Canada
8:00 pm
Calgary Intl (YYC)
8:34 pm
Vancouver Intl (YVR)
1h 34m
Cabin:Economy
Airline Fare Rules
Total reward miles 6706
Total taxes & fees $189.14