That's the biggest challenge. It's the cumulative effect over a period of time that you will notice rather than noticing an immediate difference. Smoothie is a fine complement. Most are fruit and milk so that is a good representation of 2 food groups, but probably not a good meal replacement. At least five servings of both combined.
Servings are an extremely important part a diet.
For example, a serving of a fruit or vegetable is about the size of a small fist. A serving of peanut butter is the size of a golf ball. If you get scientific, a reference person, so to speak, it is about 2000 for women and about 2500 for men. That's why they're called vitamin supplements. Vitamins they should supplement a diet, not replace it.