Yes, vat often means take, grab, or pick up, but in Afrikaans it is also used in many everyday expressions.
In vat 'n blaaskans, it means take in the same way English says take a break.
This is an idiomatic phrase, so it is best to learn it as a chunk:
You may also hear neem 'n blaaskans, but vat sounds very natural and common in speech.
'n is the Afrikaans indefinite article, so it means a or an.
Examples:
A few important points:
So vat 'n blaaskans sounds roughly like vat uh blaaskans.
Blaaskans is a compound word:
So literally, it is something like a chance to breathe.
That is why it means:
This is a nice example of how Afrikaans often builds meanings through compounds.
Afrikaans very often writes compound nouns as one word.
So:
This is similar to many Germanic languages. English sometimes does this too, but less consistently. In Afrikaans, writing the compound as one word is standard.
Kom ons vat 'n blaaskans! is natural, friendly, and fairly informal to neutral.
It would work well in:
If you wanted something more formal, you might choose a different wording, such as:
But the original sentence is perfectly normal and common.
A rough English-friendly guide would be:
kom ons fat uh BLAAS-kahns
A few notes:
This is only an approximation, but it is good enough to get started.