Can I brick up vents

Well the purpose of them when you had the joists was to ventilate the floor.

The question from me is why would you want to pour concrete instead of having a timber floor? Is there a defect that you haven't told us or advice you have been given? It is expensive and disruptive work to replace a timber floor with concrete and will have knock-on effects throughout the rest of the property. The knock-on effects could be more dampness. The airflow through the vents is designed to evaporate moisture from the compacted earth solum. Now you are sealing it, the moisture (depending on the site conditions) may saturate and present as dampness on the walls. One vital thing to do now is to ensure a 'perfect' bond between the DPM (damp proof membrane) and the DPC (damp proof course) including on internal solid walls or you risk moisture passing through any unsealed gap, however minor.

Is there a room further into the house with floorboards? If so you will still need to ventilate this from the existing vents, this can be done using pipework buried in your new concrete. Check U tube for how to lay a solid floor making sure you have a damp proof membrane also

This all sounds ok to me . Just wondering why you aren’t putting at least 100 mm floor insulation (celotex) or are you intending on putting it on top of the concrete and under the topping ?