March and April are interesting times in Canada. Much of the country is still very cold. You will find snow in most of the country in March except the very temperate parts (which is essentially the extreme West coast). Even in the warmer parts like Toronto the temperature averages around 10C in April. There might well be snow. In the central cities like Winnipeg the average temperature is below freezing, even in April. In Most parts of the country, including Vancouver, people will be still skiing in March.
Whether you will find the place 'dead' depends very much on what you want to see. If you are solely visiting cities, then most museums and other indoor attractions will be open. If you are looking for outdoor attractions then many will be closed, and many more on reduced hours. Most parks will probably not be open (or open only for winter activities), and conditions on hiking trails will be treacherous, even if open. If you are looking for winter attractions (ski resorts etc.) then they will be coming to the end of the season, but should still be open at least through March.
It's perfectly possible to have a great vacation in Canada at this time, provided you are dressed for the weather (and potential bad weather), and are not expecting outdoor activities. And as blackbird57 says, be aware that Canada is a huge country. Simply driving from Vancouver to Toronto will take a week, without allowing for any time to stop and sightsee.
If you have only a week or two, but really want to travel at this time, you might consider staying in the BC coastal area (Vancouver, Vancouver Island and Rocky Mountains). The weather is much milder there; there is more than enough to occupy you for two weeks, and the scenery is better than the prairies (sorry Saskatchewan!)
I'm also not sure who told you about the "nice landscape in the centre". The prairie provinces (Manitoba, Saskatchewan and eastern Alberta) are extremely flat. People go there for many reasons, but not often for the landscape. Although tastes may vary.