Singleness is the state out of which some, though not all, people are called to a more committed state of life, such as marriage, religious life, or Holy Orders. In this sense a lack of calling to anything else could be regarded as the absence of a vocation rather than a vocation itself. In this sense it would be false to say everyone has a vocation. We would have to say that most people do, but some do not. It is the will of God for them to remain in this default single state rather than enter any other state of life.
On the other hand, all people are called to holiness, and some people are called to work towards holiness specifically in the context of perpetual single life. In this other sense singleness could definitely be seen as a vocation. This way of looking at it has become popular in recent decades, giving rise to the idea that everyone has a vocation, including those called to stay in the single state we were all born into rather than enter any other state of life.