12 night trip to London

We are going to London “2 couples” first of April this year. Flying in
to Heathrow, staying around Victoria Station. Is there a tube line
that runs that way now?

Not direct, but there are direct coaches to Victoria.

What’s the best travel option to and from Bath from Victoria station?

Take the Underground to Paddington and a train from there.

You will no doubt find yourself needing to research lots of transportation routes during your stay. Take a look at the Transport for London app for your smartphone and the National Rail site for train options. Rick Steves does a great job on describing routes and options in his guidebook.

Getting around, and to, London will be easier with the government's guide.
It's good to start practicing its interactive route search now, on this question, so you will be ready to use it all the time you are there.
https://tfl.gov.uk/

Google Maps has a similar function although with less info about fares.

You would need to take the tube from Victoria Tube Station to Paddington Station. Then take the train from Paddington Station to Bath, which is an 1 hour 19 minutes trip. You should go on national rail.co.uk, and buy return (round trip) tickets eleven weeks in advance to get advanced fares, I looked today advanced return ticket for one is £25.00.

Do take note of Robin Z's guidance to buy well in advance. If you wait till the last moment to buy a ticket from London to Bath, you could pay over 200 pounds for the round-trip, though the off-peak return fare is 84.60 if you don't need to get an early start. But be sure of your travel plan before buying an Advance ticket.

Heathrow, staying around Victoria Station. Is there a tube line that runs that way now?

Yes, and the tube is cheap: £5.10 peak, £3.10 off peak
You take the Piccadilly line (dark blue) from whichever terminal you land at (that is the only line at Heathrow), as far as Hammersmith, then cross the platform and take a District line (green) train to Victoria. Check platform indicators to make sure you get the correct train.

What’s the best travel option to and from Bath from Victoria station?

Train from London Paddington station. From Victoria to Paddington by tube.

You need to do some research about how the tube works. The lines and about how to pay (Oyster Card). You will be using the tube many times during your stay in London.
The tube map is here: http://content.tfl.gov.uk/standard-tube-map.pdf

Bath is great, check out the Brooks Guesthouse for a super B&B.

Also, the best Fish and Chips in England are at "The Scallop Shell."