How frustrating and unfair. Anyone who is in first 6 months of a difficult new job should be given extra access to management time and support, in whatever guise its needed.
Where exactly are you being judged as underperforming? That's really crucial to understand as it could be your manager has one set of priorities and yours are not aligned to theirs (which is your manager's fault but if it's happening it is best you identify it and tackle it).
Here is what I'd do. "Proactively" book a longish catch up meeting in your manager's calendar for, say, Wednesday.
Turn up armed with, a short list of your accomplishments so far (including holding the fort while manager went on hols along with everyone else) and also a list of tasks you are working on as well as a copy of your job description.
Say to your manager you are aware that they have some criticisms of your work so far, so you want to reset and agree exactly what your manager wants you to work on improving in the next week, fortnight, month, three months. Manager can't just say "everything" is top priority so it will force manager to, erm, manage.
Also raise the subject of unexpected additional workload due to colleague leaving. Ask manager to review your recommendations for what to take OUT of your existing workload to make space for the new project you have inherited.
Offer to copy manager in on all your emails for a short period, so manager can see what you are doing.
Then ask to arrange a short catch up meeting every week to review progress against list of agreed items you are working on.
Managers like this, ie poor ones, mean unfortunately you need to be very specific with what is required, get it in writing and reviews frequently.
Give it six months, then look for another job or transfer out.